<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2020 Workiva-->
<!--r:58bd7104-74e0-40db-81ff-7ce5ac9f5a9b,g:a8740470-9635-4f62-b986-2780c7a4c22f-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://casella.com/role/COVERPAGE" xlink:type="simple" xlink:href="cwst-20200630.xsd#COVERPAGE"/>
  <link:definitionLink xlink:role="http://casella.com/role/COVERPAGE" xlink:type="extended" id="if1207ae3a09d45bb909148518f74a810_COVERPAGE">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_191bf0e5-f152-4b9b-b22c-d5ebcded8eaf" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_DocumentType_191bf0e5-f152-4b9b-b22c-d5ebcded8eaf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentQuarterlyReport_f7542a10-23e1-4bed-a7d4-87d42f1978d4" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentQuarterlyReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_DocumentQuarterlyReport_f7542a10-23e1-4bed-a7d4-87d42f1978d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentTransitionReport_e90365e9-b64d-4df5-8319-6fc14ebf81f6" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentTransitionReport"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_DocumentTransitionReport_e90365e9-b64d-4df5-8319-6fc14ebf81f6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_c379a8a4-0fb4-4b00-82b5-7bf30247954a" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityRegistrantName_c379a8a4-0fb4-4b00-82b5-7bf30247954a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_11746ac6-0b15-42ea-9c22-43e3c79434f9" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityFileNumber_11746ac6-0b15-42ea-9c22-43e3c79434f9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_80976948-401d-47db-95e0-8788f34d2cb8" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_DocumentPeriodEndDate_80976948-401d-47db-95e0-8788f34d2cb8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalYearFocus_bff22b00-5132-4d18-8e2d-78a5546c10d9" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentFiscalYearFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_DocumentFiscalYearFocus_bff22b00-5132-4d18-8e2d-78a5546c10d9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentFiscalPeriodFocus_a19c038d-ff13-4a7f-84d2-71b5825cc777" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentFiscalPeriodFocus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_DocumentFiscalPeriodFocus_a19c038d-ff13-4a7f-84d2-71b5825cc777" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CurrentFiscalYearEndDate_b2290213-d2fb-4d15-a1bc-26779790c7d9" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_CurrentFiscalYearEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_CurrentFiscalYearEndDate_b2290213-d2fb-4d15-a1bc-26779790c7d9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_cbdd5463-d675-4d72-9c42-7f4e225ecc1f" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityCentralIndexKey_cbdd5463-d675-4d72-9c42-7f4e225ecc1f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_77db5606-f8b0-45ea-9752-d0bfc4f986af" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityIncorporationStateCountryCode_77db5606-f8b0-45ea-9752-d0bfc4f986af" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_04565510-e6c5-444d-97ac-1f3adf042bd3" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityCommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_04565510-e6c5-444d-97ac-1f3adf042bd3" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFilerCategory_a84c3bb9-af25-4b20-8f84-c096eb0dca34" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityFilerCategory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityFilerCategory_a84c3bb9-af25-4b20-8f84-c096eb0dca34" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_e41a9923-b0a2-4cd1-aadd-d60b0e7e63d7" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_Security12bTitle_e41a9923-b0a2-4cd1-aadd-d60b0e7e63d7" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_2dedd9b3-5ef7-4554-ba6b-388a8f5bcb22" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_TradingSymbol_2dedd9b3-5ef7-4554-ba6b-388a8f5bcb22" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_3fbea204-8342-49b3-8c89-95f85da2dd28" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_SecurityExchangeName_3fbea204-8342-49b3-8c89-95f85da2dd28" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntitySmallBusiness_1da1608e-4ebf-4798-b2e9-2d33b4867695" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntitySmallBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntitySmallBusiness_1da1608e-4ebf-4798-b2e9-2d33b4867695" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_239128ae-6460-4f40-b021-5987b8178eb0" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityEmergingGrowthCompany_239128ae-6460-4f40-b021-5987b8178eb0" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityShellCompany_fc1ef730-7496-44bf-9e86-bd34cda7a56a" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityShellCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityShellCompany_fc1ef730-7496-44bf-9e86-bd34cda7a56a" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_49713d38-f928-4199-b69c-864aeb6eec93" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_AmendmentFlag_49713d38-f928-4199-b69c-864aeb6eec93" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCurrentReportingStatus_cfa57012-ff22-4bb2-ae9f-76ee58e80afd" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityCurrentReportingStatus"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityCurrentReportingStatus_cfa57012-ff22-4bb2-ae9f-76ee58e80afd" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityInteractiveDataCurrent_7a1979a0-33ae-4709-8543-540abcd0fd70" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityInteractiveDataCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityInteractiveDataCurrent_7a1979a0-33ae-4709-8543-540abcd0fd70" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_357fb5de-6f17-424b-a336-df3a2007ab4e" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityTaxIdentificationNumber_357fb5de-6f17-424b-a336-df3a2007ab4e" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_e70dd850-7250-4f1b-90e1-d43edb3d2038" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityAddressAddressLine1_e70dd850-7250-4f1b-90e1-d43edb3d2038" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_7c47dd48-9852-421e-bfa0-5a93835d0f90" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityAddressCityOrTown_7c47dd48-9852-421e-bfa0-5a93835d0f90" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_4f183bfa-6724-4b8d-aacb-7cbeb8eb45df" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityAddressStateOrProvince_4f183bfa-6724-4b8d-aacb-7cbeb8eb45df" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_f99db1c9-7dd9-4f59-8d2d-aea6a9538011" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_EntityAddressPostalZipCode_f99db1c9-7dd9-4f59-8d2d-aea6a9538011" xlink:type="arc" order="26"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_406f733a-1b4f-4c94-af61-68f3f629ee17" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_CityAreaCode_406f733a-1b4f-4c94-af61-68f3f629ee17" xlink:type="arc" order="27"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_9a2e8af5-942e-4e9b-b529-8d7fb9c857de" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_LocalPhoneNumber_9a2e8af5-942e-4e9b-b529-8d7fb9c857de" xlink:type="arc" order="28"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_43babcc4-8efd-4978-ae5a-19c19aa13230" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_8a465dd3-8a5f-4e6e-8f4f-270e09db4105" xlink:to="loc_dei_DocumentInformationTable_43babcc4-8efd-4978-ae5a-19c19aa13230" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_b93ad916-9de1-4e2d-a47e-b0012cecb18b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_43babcc4-8efd-4978-ae5a-19c19aa13230" xlink:to="loc_us-gaap_StatementClassOfStockAxis_b93ad916-9de1-4e2d-a47e-b0012cecb18b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_b93ad916-9de1-4e2d-a47e-b0012cecb18b_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_b93ad916-9de1-4e2d-a47e-b0012cecb18b" xlink:to="loc_us-gaap_ClassOfStockDomain_b93ad916-9de1-4e2d-a47e-b0012cecb18b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_d2b18166-9cf0-4b83-a0c8-8ce21c9c910e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_b93ad916-9de1-4e2d-a47e-b0012cecb18b" xlink:to="loc_us-gaap_ClassOfStockDomain_d2b18166-9cf0-4b83-a0c8-8ce21c9c910e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_c0a30e6e-3ba6-469e-aa1b-287eea74244c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_d2b18166-9cf0-4b83-a0c8-8ce21c9c910e" xlink:to="loc_us-gaap_CommonClassAMember_c0a30e6e-3ba6-469e-aa1b-287eea74244c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_1f19e595-9f64-4f1e-a7ed-78bdf2e5d026" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_d2b18166-9cf0-4b83-a0c8-8ce21c9c910e" xlink:to="loc_us-gaap_CommonClassBMember_1f19e595-9f64-4f1e-a7ed-78bdf2e5d026" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/CONSOLIDATEDBALANCESHEETS" xlink:type="simple" xlink:href="cwst-20200630.xsd#CONSOLIDATEDBALANCESHEETS"/>
  <link:definitionLink xlink:role="http://casella.com/role/CONSOLIDATEDBALANCESHEETS" xlink:type="extended" id="ia5632db4afe34ce380b500c9de79376b_CONSOLIDATEDBALANCESHEETS">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_136c0979-f0cb-41ca-8658-a534a9030e5f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_0ffd07a3-1313-48d4-9845-e23d172dd697" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_136c0979-f0cb-41ca-8658-a534a9030e5f" xlink:to="loc_us-gaap_AssetsAbstract_0ffd07a3-1313-48d4-9845-e23d172dd697" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_f2fc574d-581f-4d7a-8898-c2f1e56ae5f7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0ffd07a3-1313-48d4-9845-e23d172dd697" xlink:to="loc_us-gaap_AssetsCurrentAbstract_f2fc574d-581f-4d7a-8898-c2f1e56ae5f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_d9050d8f-b567-492b-92d8-52b063e0e51a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_f2fc574d-581f-4d7a-8898-c2f1e56ae5f7" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_d9050d8f-b567-492b-92d8-52b063e0e51a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_58251d9f-a701-4a2a-998a-d32ba26017df" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_f2fc574d-581f-4d7a-8898-c2f1e56ae5f7" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_58251d9f-a701-4a2a-998a-d32ba26017df" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesReceivable_70118bf0-6cfd-41f8-b857-7fea08a1cbaa" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeTaxesReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_f2fc574d-581f-4d7a-8898-c2f1e56ae5f7" xlink:to="loc_us-gaap_IncomeTaxesReceivable_70118bf0-6cfd-41f8-b857-7fea08a1cbaa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseCurrent_b1aacc5f-0abb-49ca-b8ca-c225982b3ff8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PrepaidExpenseCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_f2fc574d-581f-4d7a-8898-c2f1e56ae5f7" xlink:to="loc_us-gaap_PrepaidExpenseCurrent_b1aacc5f-0abb-49ca-b8ca-c225982b3ff8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryGross_26a4ec1e-b516-4949-a724-bf7721182264" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InventoryGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_f2fc574d-581f-4d7a-8898-c2f1e56ae5f7" xlink:to="loc_us-gaap_InventoryGross_26a4ec1e-b516-4949-a724-bf7721182264" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsCurrent_4ef58e9a-47e4-4473-8393-7b4b62290620" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_f2fc574d-581f-4d7a-8898-c2f1e56ae5f7" xlink:to="loc_us-gaap_OtherAssetsCurrent_4ef58e9a-47e4-4473-8393-7b4b62290620" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_4101a548-043d-4d63-8464-9893014d8274" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_f2fc574d-581f-4d7a-8898-c2f1e56ae5f7" xlink:to="loc_us-gaap_AssetsCurrent_4101a548-043d-4d63-8464-9893014d8274" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_d572227c-54c4-494f-a791-da06154badfa" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0ffd07a3-1313-48d4-9845-e23d172dd697" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_d572227c-54c4-494f-a791-da06154badfa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_2779f1ac-3565-4571-9ad7-a9aebb5186fd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0ffd07a3-1313-48d4-9845-e23d172dd697" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_2779f1ac-3565-4571-9ad7-a9aebb5186fd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_9f3945a9-7927-4da3-91cd-ad2f1f481e26" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0ffd07a3-1313-48d4-9845-e23d172dd697" xlink:to="loc_us-gaap_Goodwill_9f3945a9-7927-4da3-91cd-ad2f1f481e26" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_b3ca1425-c479-4bbf-b0d9-d5a63c45cda9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0ffd07a3-1313-48d4-9845-e23d172dd697" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_b3ca1425-c479-4bbf-b0d9-d5a63c45cda9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_6ad06bdc-bc3e-4719-8da7-aec0ab8290f8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestrictedCashAndInvestmentsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0ffd07a3-1313-48d4-9845-e23d172dd697" xlink:to="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_6ad06bdc-bc3e-4719-8da7-aec0ab8290f8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentsAndOtherNoncurrentAssets_6972175a-b5cf-412e-ac73-60a23ce610b1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InvestmentsAndOtherNoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0ffd07a3-1313-48d4-9845-e23d172dd697" xlink:to="loc_us-gaap_InvestmentsAndOtherNoncurrentAssets_6972175a-b5cf-412e-ac73-60a23ce610b1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxAssetsNet_041638c0-c4f8-4e5f-8ab8-1a9a24467ef7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DeferredIncomeTaxAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0ffd07a3-1313-48d4-9845-e23d172dd697" xlink:to="loc_us-gaap_DeferredIncomeTaxAssetsNet_041638c0-c4f8-4e5f-8ab8-1a9a24467ef7" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_d5338942-bbfa-4afd-82dd-5ec876cd7733" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0ffd07a3-1313-48d4-9845-e23d172dd697" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_d5338942-bbfa-4afd-82dd-5ec876cd7733" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_8b0f3327-c100-44b5-904a-be9a409d81d7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_0ffd07a3-1313-48d4-9845-e23d172dd697" xlink:to="loc_us-gaap_Assets_8b0f3327-c100-44b5-904a-be9a409d81d7" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_72ace8fd-4ac5-4517-a6bd-9835bd139ebf" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_136c0979-f0cb-41ca-8658-a534a9030e5f" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_72ace8fd-4ac5-4517-a6bd-9835bd139ebf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_64f1f248-1d35-4ab6-bf6b-66de19a56d97" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_72ace8fd-4ac5-4517-a6bd-9835bd139ebf" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_64f1f248-1d35-4ab6-bf6b-66de19a56d97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_abacb362-bcdc-4c3a-911c-c7882322f55d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_64f1f248-1d35-4ab6-bf6b-66de19a56d97" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_abacb362-bcdc-4c3a-911c-c7882322f55d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityCurrent_7b9afe51-f405-4b49-8aa6-8068b8fa4f26" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_64f1f248-1d35-4ab6-bf6b-66de19a56d97" xlink:to="loc_us-gaap_OperatingLeaseLiabilityCurrent_7b9afe51-f405-4b49-8aa6-8068b8fa4f26" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_cb11bd10-4641-465a-93da-f9e380820333" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_64f1f248-1d35-4ab6-bf6b-66de19a56d97" xlink:to="loc_us-gaap_AccountsPayableCurrent_cb11bd10-4641-465a-93da-f9e380820333" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_2588d144-0c09-4d6a-bc9b-6924c3a9bdf5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeRelatedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_64f1f248-1d35-4ab6-bf6b-66de19a56d97" xlink:to="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_2588d144-0c09-4d6a-bc9b-6924c3a9bdf5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPayableCurrent_ede4b789-b405-4f6c-b683-a57fa14d85b6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InterestPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_64f1f248-1d35-4ab6-bf6b-66de19a56d97" xlink:to="loc_us-gaap_InterestPayableCurrent_ede4b789-b405-4f6c-b683-a57fa14d85b6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_11039c10-e47f-40ae-81c4-a7548aa0400d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_64f1f248-1d35-4ab6-bf6b-66de19a56d97" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_11039c10-e47f-40ae-81c4-a7548aa0400d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCosts_9653d3e3-7d0a-4f61-813c-82cb66e7f8ad" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_64f1f248-1d35-4ab6-bf6b-66de19a56d97" xlink:to="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCosts_9653d3e3-7d0a-4f61-813c-82cb66e7f8ad" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAccruedLiabilitiesCurrent_bfadf263-0d39-470f-a076-c6e09545695b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherAccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_64f1f248-1d35-4ab6-bf6b-66de19a56d97" xlink:to="loc_us-gaap_OtherAccruedLiabilitiesCurrent_bfadf263-0d39-470f-a076-c6e09545695b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_363d60f7-3308-4486-aea4-fecba6e70d00" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_64f1f248-1d35-4ab6-bf6b-66de19a56d97" xlink:to="loc_us-gaap_LiabilitiesCurrent_363d60f7-3308-4486-aea4-fecba6e70d00" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_4c4bd156-6a28-4888-abd5-c091defdd754" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_72ace8fd-4ac5-4517-a6bd-9835bd139ebf" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_4c4bd156-6a28-4888-abd5-c091defdd754" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_fd8961eb-4365-4978-845e-19b6c83a2a05" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseLiabilityNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_72ace8fd-4ac5-4517-a6bd-9835bd139ebf" xlink:to="loc_us-gaap_OperatingLeaseLiabilityNoncurrent_fd8961eb-4365-4978-845e-19b6c83a2a05" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCostsNoncurrent_88e7fdee-46c1-4ff6-bbb7-3cd54755e6ea" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCostsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_72ace8fd-4ac5-4517-a6bd-9835bd139ebf" xlink:to="loc_us-gaap_AccruedCappingClosurePostClosureAndEnvironmentalCostsNoncurrent_88e7fdee-46c1-4ff6-bbb7-3cd54755e6ea" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_19f96328-f22e-4d65-b57c-8d11552e1328" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_72ace8fd-4ac5-4517-a6bd-9835bd139ebf" xlink:to="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_19f96328-f22e-4d65-b57c-8d11552e1328" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_11beda51-5410-4725-861e-c7393f41c835" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_72ace8fd-4ac5-4517-a6bd-9835bd139ebf" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_11beda51-5410-4725-861e-c7393f41c835" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_8645a3a4-3e3f-4bbc-9230-b34864e2f33a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommitmentsAndContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_72ace8fd-4ac5-4517-a6bd-9835bd139ebf" xlink:to="loc_us-gaap_CommitmentsAndContingencies_8645a3a4-3e3f-4bbc-9230-b34864e2f33a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_5bd05305-60e7-415a-a3e7-ad1e865f2ce1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_72ace8fd-4ac5-4517-a6bd-9835bd139ebf" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_5bd05305-60e7-415a-a3e7-ad1e865f2ce1" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_4e7ac79c-7a1b-41d7-a079-1aab1e40d43c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_5bd05305-60e7-415a-a3e7-ad1e865f2ce1" xlink:to="loc_us-gaap_StockholdersEquityAbstract_4e7ac79c-7a1b-41d7-a079-1aab1e40d43c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValue_5fef8b5e-ba7f-40c2-9cf2-12bab8206c21" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_4e7ac79c-7a1b-41d7-a079-1aab1e40d43c" xlink:to="loc_us-gaap_CommonStockValue_5fef8b5e-ba7f-40c2-9cf2-12bab8206c21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalCommonStock_b9886829-2444-49bf-9814-a090298881b8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AdditionalPaidInCapitalCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_4e7ac79c-7a1b-41d7-a079-1aab1e40d43c" xlink:to="loc_us-gaap_AdditionalPaidInCapitalCommonStock_b9886829-2444-49bf-9814-a090298881b8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_9253d440-0298-4f46-be29-040924d31ac5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_4e7ac79c-7a1b-41d7-a079-1aab1e40d43c" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_9253d440-0298-4f46-be29-040924d31ac5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_4b1abd80-0087-4af6-803d-4bc50f62fbfd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_4e7ac79c-7a1b-41d7-a079-1aab1e40d43c" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_4b1abd80-0087-4af6-803d-4bc50f62fbfd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_a8e67fc3-27be-44f7-8063-c83d5fa4ca4a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAbstract_5bd05305-60e7-415a-a3e7-ad1e865f2ce1" xlink:to="loc_us-gaap_StockholdersEquity_a8e67fc3-27be-44f7-8063-c83d5fa4ca4a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_64e7164b-0599-4980-bc42-bab53c7bc6d3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_72ace8fd-4ac5-4517-a6bd-9835bd139ebf" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_64e7164b-0599-4980-bc42-bab53c7bc6d3" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_561348b2-dfb4-418e-93be-2cc654bdb174" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_136c0979-f0cb-41ca-8658-a534a9030e5f" xlink:to="loc_us-gaap_StatementTable_561348b2-dfb4-418e-93be-2cc654bdb174" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_77672ec5-f84e-4f40-9ec5-5b493d179f4b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_561348b2-dfb4-418e-93be-2cc654bdb174" xlink:to="loc_us-gaap_StatementClassOfStockAxis_77672ec5-f84e-4f40-9ec5-5b493d179f4b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_77672ec5-f84e-4f40-9ec5-5b493d179f4b_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_77672ec5-f84e-4f40-9ec5-5b493d179f4b" xlink:to="loc_us-gaap_ClassOfStockDomain_77672ec5-f84e-4f40-9ec5-5b493d179f4b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_65ce9e61-156c-4ac5-9904-a6c89dba3a5e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_77672ec5-f84e-4f40-9ec5-5b493d179f4b" xlink:to="loc_us-gaap_ClassOfStockDomain_65ce9e61-156c-4ac5-9904-a6c89dba3a5e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_3dc2f931-f49d-4f19-bf8e-99dd3ba0aeee" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_65ce9e61-156c-4ac5-9904-a6c89dba3a5e" xlink:to="loc_us-gaap_CommonClassAMember_3dc2f931-f49d-4f19-bf8e-99dd3ba0aeee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_4e9f3d0f-b10d-4d28-b86e-ddf67ecc635d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_65ce9e61-156c-4ac5-9904-a6c89dba3a5e" xlink:to="loc_us-gaap_CommonClassBMember_4e9f3d0f-b10d-4d28-b86e-ddf67ecc635d" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/CONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="simple" xlink:href="cwst-20200630.xsd#CONSOLIDATEDBALANCESHEETSParenthetical"/>
  <link:definitionLink xlink:role="http://casella.com/role/CONSOLIDATEDBALANCESHEETSParenthetical" xlink:type="extended" id="ifcd9e25229414f9e84a9a5c62f8ec7fa_CONSOLIDATEDBALANCESHEETSParenthetical">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_f4194280-7b40-4064-9001-bdadc5bf55d0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_42a6a6ff-3564-46bc-a7cb-b81f4ed27c8a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f4194280-7b40-4064-9001-bdadc5bf55d0" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_42a6a6ff-3564-46bc-a7cb-b81f4ed27c8a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_9b7d56db-d458-42d5-8462-38abd5bd1016" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f4194280-7b40-4064-9001-bdadc5bf55d0" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_9b7d56db-d458-42d5-8462-38abd5bd1016" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_c895c4c7-add4-4552-b506-25a8ab073743" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f4194280-7b40-4064-9001-bdadc5bf55d0" xlink:to="loc_us-gaap_CommonStockSharesIssued_c895c4c7-add4-4552-b506-25a8ab073743" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_69307fa7-8176-4b64-b95d-5cda1436e1f8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f4194280-7b40-4064-9001-bdadc5bf55d0" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_69307fa7-8176-4b64-b95d-5cda1436e1f8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_StockholdersEquityVotesperShareHeld_0015ffdb-7140-407d-ab2e-624dd6e22081" xlink:href="cwst-20200630.xsd#cwst_StockholdersEquityVotesperShareHeld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_f4194280-7b40-4064-9001-bdadc5bf55d0" xlink:to="loc_cwst_StockholdersEquityVotesperShareHeld_0015ffdb-7140-407d-ab2e-624dd6e22081" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_bdaac020-a0f7-4ed9-b2b3-c3f55c6c395e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_f4194280-7b40-4064-9001-bdadc5bf55d0" xlink:to="loc_us-gaap_StatementTable_bdaac020-a0f7-4ed9-b2b3-c3f55c6c395e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_0378ac9f-6e4c-4b93-9779-1915838fc97d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_bdaac020-a0f7-4ed9-b2b3-c3f55c6c395e" xlink:to="loc_us-gaap_StatementClassOfStockAxis_0378ac9f-6e4c-4b93-9779-1915838fc97d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_0378ac9f-6e4c-4b93-9779-1915838fc97d_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_0378ac9f-6e4c-4b93-9779-1915838fc97d" xlink:to="loc_us-gaap_ClassOfStockDomain_0378ac9f-6e4c-4b93-9779-1915838fc97d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_4dc884cc-4784-441a-b2ff-067e44283d8c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_0378ac9f-6e4c-4b93-9779-1915838fc97d" xlink:to="loc_us-gaap_ClassOfStockDomain_4dc884cc-4784-441a-b2ff-067e44283d8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_2262539b-dfd1-4206-9cdf-ef204ee16389" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_4dc884cc-4784-441a-b2ff-067e44283d8c" xlink:to="loc_us-gaap_CommonClassAMember_2262539b-dfd1-4206-9cdf-ef204ee16389" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_8edefe4b-61d4-4501-bdab-5b880c026089" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_4dc884cc-4784-441a-b2ff-067e44283d8c" xlink:to="loc_us-gaap_CommonClassBMember_8edefe4b-61d4-4501-bdab-5b880c026089" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/CONSOLIDATEDSTATEMENTSOFOPERATIONSUnaudited" xlink:type="simple" xlink:href="cwst-20200630.xsd#CONSOLIDATEDSTATEMENTSOFOPERATIONSUnaudited"/>
  <link:definitionLink xlink:role="http://casella.com/role/CONSOLIDATEDSTATEMENTSOFOPERATIONSUnaudited" xlink:type="extended" id="i49f262e75e934f258952e4ab6727d69c_CONSOLIDATEDSTATEMENTSOFOPERATIONSUnaudited"/>
  <link:roleRef roleURI="http://casella.com/role/CONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMEUnaudited" xlink:type="simple" xlink:href="cwst-20200630.xsd#CONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMEUnaudited"/>
  <link:definitionLink xlink:role="http://casella.com/role/CONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMEUnaudited" xlink:type="extended" id="i2aeddcff709c425a946994adc3164e5f_CONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMEUnaudited"/>
  <link:roleRef roleURI="http://casella.com/role/CONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITYDEFICITUnaudited" xlink:type="simple" xlink:href="cwst-20200630.xsd#CONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITYDEFICITUnaudited"/>
  <link:definitionLink xlink:role="http://casella.com/role/CONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITYDEFICITUnaudited" xlink:type="extended" id="i6cffb376fe3b4b1faf323e5d2f3754ee_CONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITYDEFICITUnaudited">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_bb756cf2-7b35-44ae-9277-a29999efbed3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_76ded720-5b97-4f28-88a5-c4e8f6a2382d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_bb756cf2-7b35-44ae-9277-a29999efbed3" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_76ded720-5b97-4f28-88a5-c4e8f6a2382d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_4c2a575f-582e-4e80-a8bf-073481b499db" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_76ded720-5b97-4f28-88a5-c4e8f6a2382d" xlink:to="loc_us-gaap_StockholdersEquity_4c2a575f-582e-4e80-a8bf-073481b499db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_4c3f4a65-08be-4233-acdd-2d8d0ede11d1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_76ded720-5b97-4f28-88a5-c4e8f6a2382d" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_4c3f4a65-08be-4233-acdd-2d8d0ede11d1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_bbd4bb7e-5e97-47b1-b416-67cae68cb4e4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_76ded720-5b97-4f28-88a5-c4e8f6a2382d" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_bbd4bb7e-5e97-47b1-b416-67cae68cb4e4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_0047a83b-0bf1-4997-b913-ca4c35c715a2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_76ded720-5b97-4f28-88a5-c4e8f6a2382d" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_0047a83b-0bf1-4997-b913-ca4c35c715a2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueAcquisitions_98a8eddd-74dd-4ae2-9bff-3d21e0eb1f2a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueAcquisitions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_76ded720-5b97-4f28-88a5-c4e8f6a2382d" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueAcquisitions_98a8eddd-74dd-4ae2-9bff-3d21e0eb1f2a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_d24c1461-769b-4a7c-a895-06759446e09a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesAcquisitions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_76ded720-5b97-4f28-88a5-c4e8f6a2382d" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesAcquisitions_d24c1461-769b-4a7c-a895-06759446e09a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_04a98d0c-bba0-4480-b0d3-fd3c3b3e85b1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_76ded720-5b97-4f28-88a5-c4e8f6a2382d" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_04a98d0c-bba0-4480-b0d3-fd3c3b3e85b1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_38be13d4-3861-4413-bcd5-a5935e61c594" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_76ded720-5b97-4f28-88a5-c4e8f6a2382d" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_38be13d4-3861-4413-bcd5-a5935e61c594" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_b3e261ce-cf14-4bf9-bba9-e32e7f4f83a8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_76ded720-5b97-4f28-88a5-c4e8f6a2382d" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_b3e261ce-cf14-4bf9-bba9-e32e7f4f83a8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_f6b93107-ee4d-4641-bbd0-a65026f714ef" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_76ded720-5b97-4f28-88a5-c4e8f6a2382d" xlink:to="loc_us-gaap_NetIncomeLoss_f6b93107-ee4d-4641-bbd0-a65026f714ef" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_f6a9ec81-d50b-4c8a-9fd7-c1272d0d24fb" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_76ded720-5b97-4f28-88a5-c4e8f6a2382d" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_f6a9ec81-d50b-4c8a-9fd7-c1272d0d24fb" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_304562f6-2da2-4f5e-ad59-317a41faa593" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_9dd9e782-0d78-44fd-95e2-36617cba0d73" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_ece9fb75-2d37-4eb5-8c0d-068cc579a5e2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_bb756cf2-7b35-44ae-9277-a29999efbed3" xlink:to="loc_us-gaap_StatementTable_ece9fb75-2d37-4eb5-8c0d-068cc579a5e2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_15791181-67e8-4ea9-b2e1-620a1635cb02" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_ece9fb75-2d37-4eb5-8c0d-068cc579a5e2" xlink:to="loc_us-gaap_StatementClassOfStockAxis_15791181-67e8-4ea9-b2e1-620a1635cb02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_15791181-67e8-4ea9-b2e1-620a1635cb02_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_15791181-67e8-4ea9-b2e1-620a1635cb02" xlink:to="loc_us-gaap_ClassOfStockDomain_15791181-67e8-4ea9-b2e1-620a1635cb02_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_78fddce1-7c15-490e-b9b5-7afb9713f80b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_15791181-67e8-4ea9-b2e1-620a1635cb02" xlink:to="loc_us-gaap_ClassOfStockDomain_78fddce1-7c15-490e-b9b5-7afb9713f80b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_314e29cb-08c9-4852-a3c4-2e2314b1963a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_78fddce1-7c15-490e-b9b5-7afb9713f80b" xlink:to="loc_us-gaap_CommonClassAMember_314e29cb-08c9-4852-a3c4-2e2314b1963a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_0a2dfcc3-9ef0-4fb7-ae6b-b3065b0bbe1a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_78fddce1-7c15-490e-b9b5-7afb9713f80b" xlink:to="loc_us-gaap_CommonClassBMember_0a2dfcc3-9ef0-4fb7-ae6b-b3065b0bbe1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_4d9f5dd3-3740-4e01-93df-22e3463b101e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_ece9fb75-2d37-4eb5-8c0d-068cc579a5e2" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_4d9f5dd3-3740-4e01-93df-22e3463b101e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_4d9f5dd3-3740-4e01-93df-22e3463b101e_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_4d9f5dd3-3740-4e01-93df-22e3463b101e" xlink:to="loc_us-gaap_EquityComponentDomain_4d9f5dd3-3740-4e01-93df-22e3463b101e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_5159ffc3-4ac2-47e4-a562-8e584192156b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_4d9f5dd3-3740-4e01-93df-22e3463b101e" xlink:to="loc_us-gaap_EquityComponentDomain_5159ffc3-4ac2-47e4-a562-8e584192156b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_419f5cf7-f5e2-47ab-9c1a-ad95c37ce99a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5159ffc3-4ac2-47e4-a562-8e584192156b" xlink:to="loc_us-gaap_CommonStockMember_419f5cf7-f5e2-47ab-9c1a-ad95c37ce99a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_2d8e0f4c-3ff7-4c31-8a51-44d6357cae74" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5159ffc3-4ac2-47e4-a562-8e584192156b" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_2d8e0f4c-3ff7-4c31-8a51-44d6357cae74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_625bc421-bb21-4f29-a9dc-644796d65004" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5159ffc3-4ac2-47e4-a562-8e584192156b" xlink:to="loc_us-gaap_RetainedEarningsMember_625bc421-bb21-4f29-a9dc-644796d65004" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_b461ba0a-78bc-42db-a111-2544ff4ea573" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5159ffc3-4ac2-47e4-a562-8e584192156b" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_b461ba0a-78bc-42db-a111-2544ff4ea573" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_2b9e05ca-c6fd-47a2-9bef-2d302aded6b8" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_ece9fb75-2d37-4eb5-8c0d-068cc579a5e2" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_2b9e05ca-c6fd-47a2-9bef-2d302aded6b8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_2b9e05ca-c6fd-47a2-9bef-2d302aded6b8_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_2b9e05ca-c6fd-47a2-9bef-2d302aded6b8" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_2b9e05ca-c6fd-47a2-9bef-2d302aded6b8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_6332be69-6e26-4b52-8e59-1180f4ae4470" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_2b9e05ca-c6fd-47a2-9bef-2d302aded6b8" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_6332be69-6e26-4b52-8e59-1180f4ae4470" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_084edfd6-4bc0-4f6a-9c3b-c364c7dddb96" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_6332be69-6e26-4b52-8e59-1180f4ae4470" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_084edfd6-4bc0-4f6a-9c3b-c364c7dddb96" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/CONSOLIDATEDSTATEMENTSOFCASHFLOWSUnaudited" xlink:type="simple" xlink:href="cwst-20200630.xsd#CONSOLIDATEDSTATEMENTSOFCASHFLOWSUnaudited"/>
  <link:definitionLink xlink:role="http://casella.com/role/CONSOLIDATEDSTATEMENTSOFCASHFLOWSUnaudited" xlink:type="extended" id="i558e3a7a6d6042b3881774714f197497_CONSOLIDATEDSTATEMENTSOFCASHFLOWSUnaudited">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_7513d66d-a9df-4663-893d-43e71b4be9bf" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_a2df5a95-5fb1-4d43-b7bb-99ccb374d63a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_7513d66d-a9df-4663-893d-43e71b4be9bf" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_a2df5a95-5fb1-4d43-b7bb-99ccb374d63a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_99d65c2e-edc2-4120-80ec-06fa672f8e84" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_a2df5a95-5fb1-4d43-b7bb-99ccb374d63a" xlink:to="loc_us-gaap_ProfitLoss_99d65c2e-edc2-4120-80ec-06fa672f8e84" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d090d7d7-2270-4c91-891b-7cb2a683444d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_a2df5a95-5fb1-4d43-b7bb-99ccb374d63a" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d090d7d7-2270-4c91-891b-7cb2a683444d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_4710a2c5-fa75-4a7f-88aa-c63fd93a7628" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d090d7d7-2270-4c91-891b-7cb2a683444d" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_4710a2c5-fa75-4a7f-88aa-c63fd93a7628" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseExpense_0a049633-4f01-438c-879a-16678225b69a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d090d7d7-2270-4c91-891b-7cb2a683444d" xlink:to="loc_us-gaap_OperatingLeaseExpense_0a049633-4f01-438c-879a-16678225b69a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccretionExpenseIncludingAssetRetirementObligations_a31e8041-55d8-4ef5-8397-5874ed812ce2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccretionExpenseIncludingAssetRetirementObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d090d7d7-2270-4c91-891b-7cb2a683444d" xlink:to="loc_us-gaap_AccretionExpenseIncludingAssetRetirementObligations_a31e8041-55d8-4ef5-8397-5874ed812ce2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_702a7261-bc31-42a8-96d4-e3bb9696002e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d090d7d7-2270-4c91-891b-7cb2a683444d" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_702a7261-bc31-42a8-96d4-e3bb9696002e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_b99a41ec-154e-467e-b01a-a35e90cc55de" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d090d7d7-2270-4c91-891b-7cb2a683444d" xlink:to="loc_us-gaap_ShareBasedCompensation_b99a41ec-154e-467e-b01a-a35e90cc55de" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_27ed42b6-df1a-48ee-919d-37e7ae57262c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_GainLossOnSaleOfPropertyPlantEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d090d7d7-2270-4c91-891b-7cb2a683444d" xlink:to="loc_us-gaap_GainLossOnSaleOfPropertyPlantEquipment_27ed42b6-df1a-48ee-919d-37e7ae57262c" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SouthbridgeLandfillNonCashClosureCharge_50b6d518-0ecf-430b-8601-d75a86e01fc1" xlink:href="cwst-20200630.xsd#cwst_SouthbridgeLandfillNonCashClosureCharge"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d090d7d7-2270-4c91-891b-7cb2a683444d" xlink:to="loc_cwst_SouthbridgeLandfillNonCashClosureCharge_50b6d518-0ecf-430b-8601-d75a86e01fc1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NonCashExpenseFromAcquisitionActivities_485bb491-ffa9-4849-a2cc-e19309c25fe8" xlink:href="cwst-20200630.xsd#cwst_NonCashExpenseFromAcquisitionActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d090d7d7-2270-4c91-891b-7cb2a683444d" xlink:to="loc_cwst_NonCashExpenseFromAcquisitionActivities_485bb491-ffa9-4849-a2cc-e19309c25fe8" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_80fe0be9-1196-431a-9912-2b60a26308ad" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DeferredIncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d090d7d7-2270-4c91-891b-7cb2a683444d" xlink:to="loc_us-gaap_DeferredIncomeTaxExpenseBenefit_80fe0be9-1196-431a-9912-2b60a26308ad" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_392c982a-8474-4e4f-80b4-9183170140b9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_d090d7d7-2270-4c91-891b-7cb2a683444d" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_392c982a-8474-4e4f-80b4-9183170140b9" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_0b005b25-faf7-4b43-b7d6-294251b3e6d3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_392c982a-8474-4e4f-80b4-9183170140b9" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_0b005b25-faf7-4b43-b7d6-294251b3e6d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_IncreaseDecreaseInLandfillOperatingLeaseContractExpenditures_d6824609-f294-4862-9fbc-0cb8f6a00951" xlink:href="cwst-20200630.xsd#cwst_IncreaseDecreaseInLandfillOperatingLeaseContractExpenditures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_392c982a-8474-4e4f-80b4-9183170140b9" xlink:to="loc_cwst_IncreaseDecreaseInLandfillOperatingLeaseContractExpenditures_d6824609-f294-4862-9fbc-0cb8f6a00951" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayable_43eea304-f60d-4a09-803e-86e09c5bbe2c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncreaseDecreaseInAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_392c982a-8474-4e4f-80b4-9183170140b9" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayable_43eea304-f60d-4a09-803e-86e09c5bbe2c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_ed180b02-0ba2-40ce-a1a9-5a9fff517459" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_392c982a-8474-4e4f-80b4-9183170140b9" xlink:to="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_ed180b02-0ba2-40ce-a1a9-5a9fff517459" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedLiabilities_00fd65e0-1608-4809-891b-c0a0e7170c1e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncreaseDecreaseInAccruedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_392c982a-8474-4e4f-80b4-9183170140b9" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedLiabilities_00fd65e0-1608-4809-891b-c0a0e7170c1e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_b0d2c02c-b8f3-48aa-ac0e-f52e5be9269a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_a2df5a95-5fb1-4d43-b7bb-99ccb374d63a" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_b0d2c02c-b8f3-48aa-ac0e-f52e5be9269a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_167f926e-f0e4-4f36-ab1e-b34b5c2de8b4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_7513d66d-a9df-4663-893d-43e71b4be9bf" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_167f926e-f0e4-4f36-ab1e-b34b5c2de8b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_5bf44fbb-9803-43b3-9f67-f260449fc8a0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_167f926e-f0e4-4f36-ab1e-b34b5c2de8b4" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_5bf44fbb-9803-43b3-9f67-f260449fc8a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_5236356b-2b0e-4755-a5c4-27f23074ee56" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_167f926e-f0e4-4f36-ab1e-b34b5c2de8b4" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_5236356b-2b0e-4755-a5c4-27f23074ee56" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment_87691c1b-81ba-443a-b6cf-cbbd53b5c713" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_167f926e-f0e4-4f36-ab1e-b34b5c2de8b4" xlink:to="loc_us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment_87691c1b-81ba-443a-b6cf-cbbd53b5c713" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_42e4fd7d-c2c7-4800-a07d-57d1989ef08e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_167f926e-f0e4-4f36-ab1e-b34b5c2de8b4" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_42e4fd7d-c2c7-4800-a07d-57d1989ef08e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_ceab1729-1b0c-42ca-8564-e3ae3995149f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_7513d66d-a9df-4663-893d-43e71b4be9bf" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_ceab1729-1b0c-42ca-8564-e3ae3995149f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebtAndCapitalSecuritiesNet_02491011-cf76-401c-8257-3e2b62fa4959" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ProceedsFromIssuanceOfLongTermDebtAndCapitalSecuritiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_ceab1729-1b0c-42ca-8564-e3ae3995149f" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebtAndCapitalSecuritiesNet_02491011-cf76-401c-8257-3e2b62fa4959" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLongTermDebtAndCapitalSecurities_cea61332-3c33-4f04-bc8f-3cb78ddeb2cd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RepaymentsOfLongTermDebtAndCapitalSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_ceab1729-1b0c-42ca-8564-e3ae3995149f" xlink:to="loc_us-gaap_RepaymentsOfLongTermDebtAndCapitalSecurities_cea61332-3c33-4f04-bc8f-3cb78ddeb2cd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDebtIssuanceCosts_bc54f8de-dbf2-4689-9959-f2b9cce812f9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PaymentsOfDebtIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_ceab1729-1b0c-42ca-8564-e3ae3995149f" xlink:to="loc_us-gaap_PaymentsOfDebtIssuanceCosts_bc54f8de-dbf2-4689-9959-f2b9cce812f9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlansIncludingStockOptions_0596026b-dbe0-4378-bb29-583f91d64476" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlansIncludingStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_ceab1729-1b0c-42ca-8564-e3ae3995149f" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlansIncludingStockOptions_0596026b-dbe0-4378-bb29-583f91d64476" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_732d763f-a39b-4b6c-a1c1-f961cd9303d3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_ceab1729-1b0c-42ca-8564-e3ae3995149f" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_732d763f-a39b-4b6c-a1c1-f961cd9303d3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_d04a7203-c06c-4d29-a7ac-2a115b8ca275" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_ceab1729-1b0c-42ca-8564-e3ae3995149f" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_d04a7203-c06c-4d29-a7ac-2a115b8ca275" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_9ab60d92-0ce7-4b3f-aefb-4d1fa1c83653" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_7513d66d-a9df-4663-893d-43e71b4be9bf" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_9ab60d92-0ce7-4b3f-aefb-4d1fa1c83653" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_c8dfb8c2-9d68-49f8-b39b-d42612ba19d5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_7513d66d-a9df-4663-893d-43e71b4be9bf" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_c8dfb8c2-9d68-49f8-b39b-d42612ba19d5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_223c08dd-61d6-41c8-9b4e-577922bc889f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplementalCashFlowInformationAbstract_a68a3914-69d8-4cce-ada1-c4a7b0743abf" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SupplementalCashFlowInformationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_7513d66d-a9df-4663-893d-43e71b4be9bf" xlink:to="loc_us-gaap_SupplementalCashFlowInformationAbstract_a68a3914-69d8-4cce-ada1-c4a7b0743abf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SupplementalCashFlowInformationCashPaidAbstract_666eb0ac-3a95-4563-a6cb-d2bdc93e695d" xlink:href="cwst-20200630.xsd#cwst_SupplementalCashFlowInformationCashPaidAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_a68a3914-69d8-4cce-ada1-c4a7b0743abf" xlink:to="loc_cwst_SupplementalCashFlowInformationCashPaidAbstract_666eb0ac-3a95-4563-a6cb-d2bdc93e695d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPaidNet_d151c116-8aa0-4be3-843f-85c814cfb76a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InterestPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SupplementalCashFlowInformationCashPaidAbstract_666eb0ac-3a95-4563-a6cb-d2bdc93e695d" xlink:to="loc_us-gaap_InterestPaidNet_d151c116-8aa0-4be3-843f-85c814cfb76a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaidNet_81fb1ffe-9705-425f-9501-4501e21dd3a6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeTaxesPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SupplementalCashFlowInformationCashPaidAbstract_666eb0ac-3a95-4563-a6cb-d2bdc93e695d" xlink:to="loc_us-gaap_IncomeTaxesPaidNet_81fb1ffe-9705-425f-9501-4501e21dd3a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_8fe48273-c17a-4114-947a-2543beebea87" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_7513d66d-a9df-4663-893d-43e71b4be9bf" xlink:to="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_8fe48273-c17a-4114-947a-2543beebea87" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueOfAssetsAcquired_6f86219f-27b5-443c-b7a6-bc586b44764a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueOfAssetsAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_8fe48273-c17a-4114-947a-2543beebea87" xlink:to="loc_us-gaap_FairValueOfAssetsAcquired_6f86219f-27b5-443c-b7a6-bc586b44764a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_46707e18-09dc-4232-a47e-7e38cd9f4502" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_7513d66d-a9df-4663-893d-43e71b4be9bf" xlink:to="loc_us-gaap_StatementTable_46707e18-09dc-4232-a47e-7e38cd9f4502" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_b04cfe60-1167-477a-a9f8-e6445696fe95" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_46707e18-09dc-4232-a47e-7e38cd9f4502" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_b04cfe60-1167-477a-a9f8-e6445696fe95" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_b04cfe60-1167-477a-a9f8-e6445696fe95_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_b04cfe60-1167-477a-a9f8-e6445696fe95" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_b04cfe60-1167-477a-a9f8-e6445696fe95_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9c9f4960-e263-4096-95b8-83e67c69de8a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_b04cfe60-1167-477a-a9f8-e6445696fe95" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9c9f4960-e263-4096-95b8-83e67c69de8a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandfillMember_7adad369-9552-4b45-8a83-f0ba15f9606c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LandfillMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_9c9f4960-e263-4096-95b8-83e67c69de8a" xlink:to="loc_us-gaap_LandfillMember_7adad369-9552-4b45-8a83-f0ba15f9606c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/BASISOFPRESENTATION" xlink:type="simple" xlink:href="cwst-20200630.xsd#BASISOFPRESENTATION"/>
  <link:definitionLink xlink:role="http://casella.com/role/BASISOFPRESENTATION" xlink:type="extended" id="ib76ba57056e24dbbaa83d334f8f8ac51_BASISOFPRESENTATION"/>
  <link:roleRef roleURI="http://casella.com/role/BASISOFPRESENTATIONDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#BASISOFPRESENTATIONDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/BASISOFPRESENTATIONDetails" xlink:type="extended" id="i9cbb490ac35440e0b782076a7cd21fcb_BASISOFPRESENTATIONDetails"/>
  <link:roleRef roleURI="http://casella.com/role/ACCOUNTINGCHANGES" xlink:type="simple" xlink:href="cwst-20200630.xsd#ACCOUNTINGCHANGES"/>
  <link:definitionLink xlink:role="http://casella.com/role/ACCOUNTINGCHANGES" xlink:type="extended" id="idc9b767c5cac4e0a8d1e9d7e0561b71d_ACCOUNTINGCHANGES"/>
  <link:roleRef roleURI="http://casella.com/role/ACCOUNTINGCHANGESPolicies" xlink:type="simple" xlink:href="cwst-20200630.xsd#ACCOUNTINGCHANGESPolicies"/>
  <link:definitionLink xlink:role="http://casella.com/role/ACCOUNTINGCHANGESPolicies" xlink:type="extended" id="i928bc74f9cf148a09ff80a7f6d5cc875_ACCOUNTINGCHANGESPolicies"/>
  <link:roleRef roleURI="http://casella.com/role/ACCOUNTINGCHANGESTables" xlink:type="simple" xlink:href="cwst-20200630.xsd#ACCOUNTINGCHANGESTables"/>
  <link:definitionLink xlink:role="http://casella.com/role/ACCOUNTINGCHANGESTables" xlink:type="extended" id="i0ec960beb997455ca820aa049bbcb9fa_ACCOUNTINGCHANGESTables"/>
  <link:roleRef roleURI="http://casella.com/role/ACCOUNTINGCHANGESDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#ACCOUNTINGCHANGESDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/ACCOUNTINGCHANGESDetails" xlink:type="extended" id="i3710ce9c1526423bb999517e3cb25472_ACCOUNTINGCHANGESDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_adb6c3d7-d631-4981-9691-7e305bc3e5b8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountingStandardsUpdateExtensibleList_afec2edd-01e5-4b38-bab2-ad6323745652" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccountingStandardsUpdateExtensibleList"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_adb6c3d7-d631-4981-9691-7e305bc3e5b8" xlink:to="loc_us-gaap_AccountingStandardsUpdateExtensibleList_afec2edd-01e5-4b38-bab2-ad6323745652" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_04f1a4e7-1493-4144-8979-906a8a74d571" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_adb6c3d7-d631-4981-9691-7e305bc3e5b8" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_04f1a4e7-1493-4144-8979-906a8a74d571" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_2c38bdfe-1cef-4271-8be7-2524fc5dd925" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_adb6c3d7-d631-4981-9691-7e305bc3e5b8" xlink:to="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_2c38bdfe-1cef-4271-8be7-2524fc5dd925" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_b46b7e24-5524-443f-b558-ad3a78a5c612" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_2c38bdfe-1cef-4271-8be7-2524fc5dd925" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_b46b7e24-5524-443f-b558-ad3a78a5c612" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_b46b7e24-5524-443f-b558-ad3a78a5c612_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_b46b7e24-5524-443f-b558-ad3a78a5c612" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_b46b7e24-5524-443f-b558-ad3a78a5c612_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_e9597bfb-813f-4e7d-9592-398c24fa3a54" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_b46b7e24-5524-443f-b558-ad3a78a5c612" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_e9597bfb-813f-4e7d-9592-398c24fa3a54" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_4d9ab084-6a46-4b1e-abd3-59063ecea1d8" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_e9597bfb-813f-4e7d-9592-398c24fa3a54" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_4d9ab084-6a46-4b1e-abd3-59063ecea1d8" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/REVENUERECOGNITION" xlink:type="simple" xlink:href="cwst-20200630.xsd#REVENUERECOGNITION"/>
  <link:definitionLink xlink:role="http://casella.com/role/REVENUERECOGNITION" xlink:type="extended" id="i512b959b77f346ef8a22d98e71dffa46_REVENUERECOGNITION"/>
  <link:roleRef roleURI="http://casella.com/role/REVENUERECOGNITIONTables" xlink:type="simple" xlink:href="cwst-20200630.xsd#REVENUERECOGNITIONTables"/>
  <link:definitionLink xlink:role="http://casella.com/role/REVENUERECOGNITIONTables" xlink:type="extended" id="i8b03a263cf8d452f805e63c005ee5913_REVENUERECOGNITIONTables"/>
  <link:roleRef roleURI="http://casella.com/role/REVENUERECOGNITIONDisaggregationofRevenueDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#REVENUERECOGNITIONDisaggregationofRevenueDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/REVENUERECOGNITIONDisaggregationofRevenueDetails" xlink:type="extended" id="i8ff72668482942838d28178157e5093e_REVENUERECOGNITIONDisaggregationofRevenueDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_7109c282-b74e-4e06-8a1a-6e52724fd7ff" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_3971ba67-40b6-442a-b874-bf93538a5787" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_7109c282-b74e-4e06-8a1a-6e52724fd7ff" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_3971ba67-40b6-442a-b874-bf93538a5787" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_09fcae4e-a33e-43ac-b9e7-0eb517d7da86" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_7109c282-b74e-4e06-8a1a-6e52724fd7ff" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_09fcae4e-a33e-43ac-b9e7-0eb517d7da86" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_22f276fb-5475-47e9-966d-7c7a6c698ca2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_09fcae4e-a33e-43ac-b9e7-0eb517d7da86" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_22f276fb-5475-47e9-966d-7c7a6c698ca2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_22f276fb-5475-47e9-966d-7c7a6c698ca2_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_22f276fb-5475-47e9-966d-7c7a6c698ca2" xlink:to="loc_us-gaap_SegmentDomain_22f276fb-5475-47e9-966d-7c7a6c698ca2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_a58b6b74-f674-4ee0-80b6-77641a5ac529" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_22f276fb-5475-47e9-966d-7c7a6c698ca2" xlink:to="loc_us-gaap_SegmentDomain_a58b6b74-f674-4ee0-80b6-77641a5ac529" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_EasternRegionMember_73b16a92-ab05-44e6-be2b-af27814728b5" xlink:href="cwst-20200630.xsd#cwst_EasternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_a58b6b74-f674-4ee0-80b6-77641a5ac529" xlink:to="loc_cwst_EasternRegionMember_73b16a92-ab05-44e6-be2b-af27814728b5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_5d6f2c06-bc6f-4381-a4f4-b537e67ed809" xlink:href="cwst-20200630.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_a58b6b74-f674-4ee0-80b6-77641a5ac529" xlink:to="loc_cwst_WesternRegionMember_5d6f2c06-bc6f-4381-a4f4-b537e67ed809" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_f5e03a25-fcd3-4d29-ac7d-b835e8bfbd6b" xlink:href="cwst-20200630.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_a58b6b74-f674-4ee0-80b6-77641a5ac529" xlink:to="loc_cwst_ResourceSolutionsRegionMember_f5e03a25-fcd3-4d29-ac7d-b835e8bfbd6b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_730e8dd2-befe-4e4b-8f1f-a88d3deb3e03" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_09fcae4e-a33e-43ac-b9e7-0eb517d7da86" xlink:to="loc_srt_ProductOrServiceAxis_730e8dd2-befe-4e4b-8f1f-a88d3deb3e03" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_730e8dd2-befe-4e4b-8f1f-a88d3deb3e03_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_730e8dd2-befe-4e4b-8f1f-a88d3deb3e03" xlink:to="loc_srt_ProductsAndServicesDomain_730e8dd2-befe-4e4b-8f1f-a88d3deb3e03_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_7cefb79f-80e0-4cfe-ad77-c0ce1f6dccc4" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_730e8dd2-befe-4e4b-8f1f-a88d3deb3e03" xlink:to="loc_srt_ProductsAndServicesDomain_7cefb79f-80e0-4cfe-ad77-c0ce1f6dccc4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CollectionMember_19d4c2cc-8cb7-4bc8-94f5-6e3c3deaa6a9" xlink:href="cwst-20200630.xsd#cwst_CollectionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_7cefb79f-80e0-4cfe-ad77-c0ce1f6dccc4" xlink:to="loc_cwst_CollectionMember_19d4c2cc-8cb7-4bc8-94f5-6e3c3deaa6a9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LandfillRevenueMember_c0cab725-1533-4b06-a50f-3f7724eaf521" xlink:href="cwst-20200630.xsd#cwst_LandfillRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_7cefb79f-80e0-4cfe-ad77-c0ce1f6dccc4" xlink:to="loc_cwst_LandfillRevenueMember_c0cab725-1533-4b06-a50f-3f7724eaf521" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TransferMember_f956631b-8e3f-4edc-87d5-2a0fddaa2714" xlink:href="cwst-20200630.xsd#cwst_TransferMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_7cefb79f-80e0-4cfe-ad77-c0ce1f6dccc4" xlink:to="loc_cwst_TransferMember_f956631b-8e3f-4edc-87d5-2a0fddaa2714" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CustomerSolutionsMember_a57ffdfe-34c3-467f-9006-cc5b6b6055d4" xlink:href="cwst-20200630.xsd#cwst_CustomerSolutionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_7cefb79f-80e0-4cfe-ad77-c0ce1f6dccc4" xlink:to="loc_cwst_CustomerSolutionsMember_a57ffdfe-34c3-467f-9006-cc5b6b6055d4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RecyclingRevenuesMember_c2dca0d2-e5eb-4544-a4a8-79e1e9d0f0e1" xlink:href="cwst-20200630.xsd#cwst_RecyclingRevenuesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_7cefb79f-80e0-4cfe-ad77-c0ce1f6dccc4" xlink:to="loc_cwst_RecyclingRevenuesMember_c2dca0d2-e5eb-4544-a4a8-79e1e9d0f0e1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_OrganicsMember_ea52a429-13fd-4945-bd72-8a109554f052" xlink:href="cwst-20200630.xsd#cwst_OrganicsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_7cefb79f-80e0-4cfe-ad77-c0ce1f6dccc4" xlink:to="loc_cwst_OrganicsMember_ea52a429-13fd-4945-bd72-8a109554f052" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TransportationMember_da71d983-e58d-48b7-ba61-454f29456bd8" xlink:href="cwst-20200630.xsd#cwst_TransportationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_7cefb79f-80e0-4cfe-ad77-c0ce1f6dccc4" xlink:to="loc_cwst_TransportationMember_da71d983-e58d-48b7-ba61-454f29456bd8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LandfillGasToEnergyMember_7a668f9a-b822-482e-b5f3-2b853bd512db" xlink:href="cwst-20200630.xsd#cwst_LandfillGasToEnergyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_7cefb79f-80e0-4cfe-ad77-c0ce1f6dccc4" xlink:to="loc_cwst_LandfillGasToEnergyMember_7a668f9a-b822-482e-b5f3-2b853bd512db" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_fc18237e-9a6d-4ac0-97fb-23b7c12b01e0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_09fcae4e-a33e-43ac-b9e7-0eb517d7da86" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_fc18237e-9a6d-4ac0-97fb-23b7c12b01e0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_fc18237e-9a6d-4ac0-97fb-23b7c12b01e0_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_fc18237e-9a6d-4ac0-97fb-23b7c12b01e0" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_fc18237e-9a6d-4ac0-97fb-23b7c12b01e0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_0176b0cb-6861-4211-9116-206ce8d6bb57" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_fc18237e-9a6d-4ac0-97fb-23b7c12b01e0" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_0176b0cb-6861-4211-9116-206ce8d6bb57" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredAtPointInTimeMember_ef0c25fb-5d96-47c4-8844-4e4dbf49d5df" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TransferredAtPointInTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_0176b0cb-6861-4211-9116-206ce8d6bb57" xlink:to="loc_us-gaap_TransferredAtPointInTimeMember_ef0c25fb-5d96-47c4-8844-4e4dbf49d5df" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredOverTimeMember_8c373b3b-5bd6-44a2-9fea-449c1be5b21f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TransferredOverTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_0176b0cb-6861-4211-9116-206ce8d6bb57" xlink:to="loc_us-gaap_TransferredOverTimeMember_8c373b3b-5bd6-44a2-9fea-449c1be5b21f" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/REVENUERECOGNITIONNarrativeDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#REVENUERECOGNITIONNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/REVENUERECOGNITIONNarrativeDetails" xlink:type="extended" id="i86b21224820645b9bbb29885c35f6ab6_REVENUERECOGNITIONNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueInitialApplicationPeriodCumulativeEffectTransitionLineItems_a161a594-120b-4ec7-9b2a-53ce0eaf7b57" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueInitialApplicationPeriodCumulativeEffectTransitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_b4cfa34a-6245-4d66-8818-a01b4d917c36" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueInitialApplicationPeriodCumulativeEffectTransitionLineItems_a161a594-120b-4ec7-9b2a-53ce0eaf7b57" xlink:to="loc_us-gaap_Revenues_b4cfa34a-6245-4d66-8818-a01b4d917c36" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod_9050cc72-b81a-459d-88ec-ee1677c6a482" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueInitialApplicationPeriodCumulativeEffectTransitionLineItems_a161a594-120b-4ec7-9b2a-53ce0eaf7b57" xlink:to="loc_us-gaap_ContractWithCustomerPerformanceObligationSatisfiedInPreviousPeriod_9050cc72-b81a-459d-88ec-ee1677c6a482" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_deb549f1-183e-4fdf-97f0-30250e28ae25" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ContractWithCustomerAssetGrossCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueInitialApplicationPeriodCumulativeEffectTransitionLineItems_a161a594-120b-4ec7-9b2a-53ce0eaf7b57" xlink:to="loc_us-gaap_ContractWithCustomerAssetGrossCurrent_deb549f1-183e-4fdf-97f0-30250e28ae25" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_e432c82b-974f-41a7-b9c5-01949ac16f83" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ContractWithCustomerLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueInitialApplicationPeriodCumulativeEffectTransitionLineItems_a161a594-120b-4ec7-9b2a-53ce0eaf7b57" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_e432c82b-974f-41a7-b9c5-01949ac16f83" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueInitialApplicationPeriodCumulativeEffectTransitionTable_0b603722-afbd-405f-a15d-269cf15d927a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevenueInitialApplicationPeriodCumulativeEffectTransitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueInitialApplicationPeriodCumulativeEffectTransitionLineItems_a161a594-120b-4ec7-9b2a-53ce0eaf7b57" xlink:to="loc_us-gaap_RevenueInitialApplicationPeriodCumulativeEffectTransitionTable_0b603722-afbd-405f-a15d-269cf15d927a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InitialApplicationPeriodCumulativeEffectTransitionAxis_82697616-8e2e-4204-a65f-a1d3c834aa61" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InitialApplicationPeriodCumulativeEffectTransitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueInitialApplicationPeriodCumulativeEffectTransitionTable_0b603722-afbd-405f-a15d-269cf15d927a" xlink:to="loc_us-gaap_InitialApplicationPeriodCumulativeEffectTransitionAxis_82697616-8e2e-4204-a65f-a1d3c834aa61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InitialApplicationPeriodCumulativeEffectTransitionDomain_82697616-8e2e-4204-a65f-a1d3c834aa61_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InitialApplicationPeriodCumulativeEffectTransitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_InitialApplicationPeriodCumulativeEffectTransitionAxis_82697616-8e2e-4204-a65f-a1d3c834aa61" xlink:to="loc_us-gaap_InitialApplicationPeriodCumulativeEffectTransitionDomain_82697616-8e2e-4204-a65f-a1d3c834aa61_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InitialApplicationPeriodCumulativeEffectTransitionDomain_a9b15349-2580-4b1f-ba44-a0233d567bd1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InitialApplicationPeriodCumulativeEffectTransitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_InitialApplicationPeriodCumulativeEffectTransitionAxis_82697616-8e2e-4204-a65f-a1d3c834aa61" xlink:to="loc_us-gaap_InitialApplicationPeriodCumulativeEffectTransitionDomain_a9b15349-2580-4b1f-ba44-a0233d567bd1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DifferenceBetweenRevenueGuidanceInEffectBeforeAndAfterTopic606Member_5aac0365-ced8-4bff-b68c-ed9dfa7b087e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DifferenceBetweenRevenueGuidanceInEffectBeforeAndAfterTopic606Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InitialApplicationPeriodCumulativeEffectTransitionDomain_a9b15349-2580-4b1f-ba44-a0233d567bd1" xlink:to="loc_us-gaap_DifferenceBetweenRevenueGuidanceInEffectBeforeAndAfterTopic606Member_5aac0365-ced8-4bff-b68c-ed9dfa7b087e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_a8080243-98b2-4a12-964d-0fdbc315fc55" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueInitialApplicationPeriodCumulativeEffectTransitionTable_0b603722-afbd-405f-a15d-269cf15d927a" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_a8080243-98b2-4a12-964d-0fdbc315fc55" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_a8080243-98b2-4a12-964d-0fdbc315fc55_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_a8080243-98b2-4a12-964d-0fdbc315fc55" xlink:to="loc_us-gaap_TypeOfAdoptionMember_a8080243-98b2-4a12-964d-0fdbc315fc55_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_e4d64b9a-e1ec-4ad3-8e9f-5a70db27a971" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_a8080243-98b2-4a12-964d-0fdbc315fc55" xlink:to="loc_us-gaap_TypeOfAdoptionMember_e4d64b9a-e1ec-4ad3-8e9f-5a70db27a971" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountingStandardsUpdate201409Member_c8b5ac70-cecf-468d-b751-7de24089a667" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccountingStandardsUpdate201409Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_e4d64b9a-e1ec-4ad3-8e9f-5a70db27a971" xlink:to="loc_us-gaap_AccountingStandardsUpdate201409Member_c8b5ac70-cecf-468d-b751-7de24089a667" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/BUSINESSCOMBINATIONS" xlink:type="simple" xlink:href="cwst-20200630.xsd#BUSINESSCOMBINATIONS"/>
  <link:definitionLink xlink:role="http://casella.com/role/BUSINESSCOMBINATIONS" xlink:type="extended" id="icaf3da365b5e475e9389993df509eb91_BUSINESSCOMBINATIONS"/>
  <link:roleRef roleURI="http://casella.com/role/BUSINESSCOMBINATIONSTables" xlink:type="simple" xlink:href="cwst-20200630.xsd#BUSINESSCOMBINATIONSTables"/>
  <link:definitionLink xlink:role="http://casella.com/role/BUSINESSCOMBINATIONSTables" xlink:type="extended" id="i1dc22496b6ab42e8959db0a5ca249361_BUSINESSCOMBINATIONSTables"/>
  <link:roleRef roleURI="http://casella.com/role/BUSINESSCOMBINATIONSNarrativeDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#BUSINESSCOMBINATIONSNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/BUSINESSCOMBINATIONSNarrativeDetails" xlink:type="extended" id="i4fc44056bf414b99a7335acf83179c25_BUSINESSCOMBINATIONSNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_9753704b-a8df-4e92-b241-e0092b0bd9dd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfBusinessesAcquired_cbd257aa-5552-4fca-9abf-b9ac28c27219" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NumberOfBusinessesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9753704b-a8df-4e92-b241-e0092b0bd9dd" xlink:to="loc_us-gaap_NumberOfBusinessesAcquired_cbd257aa-5552-4fca-9abf-b9ac28c27219" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_71a0b4b9-7160-4931-8159-0ca539a0c06d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9753704b-a8df-4e92-b241-e0092b0bd9dd" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_71a0b4b9-7160-4931-8159-0ca539a0c06d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_ebaa7167-ce36-43c5-a8cb-694276c17bef" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9753704b-a8df-4e92-b241-e0092b0bd9dd" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_ebaa7167-ce36-43c5-a8cb-694276c17bef" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_6cf37734-df10-4d64-a27a-945725d1a699" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_ebaa7167-ce36-43c5-a8cb-694276c17bef" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_6cf37734-df10-4d64-a27a-945725d1a699" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_6cf37734-df10-4d64-a27a-945725d1a699_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_6cf37734-df10-4d64-a27a-945725d1a699" xlink:to="loc_us-gaap_SegmentDomain_6cf37734-df10-4d64-a27a-945725d1a699_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_2f823124-0a59-4990-9b7f-c1da3de52aa9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_6cf37734-df10-4d64-a27a-945725d1a699" xlink:to="loc_us-gaap_SegmentDomain_2f823124-0a59-4990-9b7f-c1da3de52aa9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_15179751-c1fa-407a-876d-20dad01c4839" xlink:href="cwst-20200630.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_2f823124-0a59-4990-9b7f-c1da3de52aa9" xlink:to="loc_cwst_WesternRegionMember_15179751-c1fa-407a-876d-20dad01c4839" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_0d0df7d0-2400-435c-bbfb-f283e7a8ebec" xlink:href="cwst-20200630.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_2f823124-0a59-4990-9b7f-c1da3de52aa9" xlink:to="loc_cwst_ResourceSolutionsRegionMember_0d0df7d0-2400-435c-bbfb-f283e7a8ebec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_EasternRegionMember_2c3e2e6f-d32b-45dd-bc53-965cc23f37c3" xlink:href="cwst-20200630.xsd#cwst_EasternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_2f823124-0a59-4990-9b7f-c1da3de52aa9" xlink:to="loc_cwst_EasternRegionMember_2c3e2e6f-d32b-45dd-bc53-965cc23f37c3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_373e2afb-02c5-41d4-849f-2fee04a3e124" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_ebaa7167-ce36-43c5-a8cb-694276c17bef" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_373e2afb-02c5-41d4-849f-2fee04a3e124" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_373e2afb-02c5-41d4-849f-2fee04a3e124_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_373e2afb-02c5-41d4-849f-2fee04a3e124" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_373e2afb-02c5-41d4-849f-2fee04a3e124_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_21a93299-eb0b-48dc-b681-3343fd32c1f3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_373e2afb-02c5-41d4-849f-2fee04a3e124" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_21a93299-eb0b-48dc-b681-3343fd32c1f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TuckinSolidWasteCollectionBusinessMember_63e58662-7135-4dcb-b2e9-71a62c83d1bc" xlink:href="cwst-20200630.xsd#cwst_TuckinSolidWasteCollectionBusinessMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_21a93299-eb0b-48dc-b681-3343fd32c1f3" xlink:to="loc_cwst_TuckinSolidWasteCollectionBusinessMember_63e58662-7135-4dcb-b2e9-71a62c83d1bc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RecyclingOperationMember_31e0224f-1a1d-408b-a7c6-ad5386eb4cac" xlink:href="cwst-20200630.xsd#cwst_RecyclingOperationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_21a93299-eb0b-48dc-b681-3343fd32c1f3" xlink:to="loc_cwst_RecyclingOperationMember_31e0224f-1a1d-408b-a7c6-ad5386eb4cac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_3e1aed5b-cd2c-45cc-a3a8-4c9f45d294f0" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_ebaa7167-ce36-43c5-a8cb-694276c17bef" xlink:to="loc_srt_RangeAxis_3e1aed5b-cd2c-45cc-a3a8-4c9f45d294f0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_3e1aed5b-cd2c-45cc-a3a8-4c9f45d294f0_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_3e1aed5b-cd2c-45cc-a3a8-4c9f45d294f0" xlink:to="loc_srt_RangeMember_3e1aed5b-cd2c-45cc-a3a8-4c9f45d294f0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_c306bbe1-5356-497a-bd04-3288ec7490fb" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_3e1aed5b-cd2c-45cc-a3a8-4c9f45d294f0" xlink:to="loc_srt_RangeMember_c306bbe1-5356-497a-bd04-3288ec7490fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_6499e8ab-afcc-4c5d-9247-3df792575235" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_c306bbe1-5356-497a-bd04-3288ec7490fb" xlink:to="loc_srt_MinimumMember_6499e8ab-afcc-4c5d-9247-3df792575235" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_71ca53f1-8b59-4337-b9a5-2f18d7394d41" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_c306bbe1-5356-497a-bd04-3288ec7490fb" xlink:to="loc_srt_MaximumMember_71ca53f1-8b59-4337-b9a5-2f18d7394d41" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/BUSINESSCOMBINATIONSSummaryofPurchasePricePaidforAcquisitionsDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#BUSINESSCOMBINATIONSSummaryofPurchasePricePaidforAcquisitionsDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/BUSINESSCOMBINATIONSSummaryofPurchasePricePaidforAcquisitionsDetail" xlink:type="extended" id="i84ebb95cc79644b0b3cb75f9c3af667d_BUSINESSCOMBINATIONSSummaryofPurchasePricePaidforAcquisitionsDetail">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_2b7cf2d5-e677-49b1-bf42-09307758921a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_8a5ac7d5-0d7d-49bf-8562-8ed22d1c2bbe" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferredAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2b7cf2d5-e677-49b1-bf42-09307758921a" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_8a5ac7d5-0d7d-49bf-8562-8ed22d1c2bbe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_af2351d6-65d9-4e90-b89b-f7130ca20b3d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_8a5ac7d5-0d7d-49bf-8562-8ed22d1c2bbe" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_af2351d6-65d9-4e90-b89b-f7130ca20b3d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_BusinessCombinationConsiderationTransferredNotesPayable_a6e9ec39-aa49-449a-88c3-b6e39ca1a6f0" xlink:href="cwst-20200630.xsd#cwst_BusinessCombinationConsiderationTransferredNotesPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_8a5ac7d5-0d7d-49bf-8562-8ed22d1c2bbe" xlink:to="loc_cwst_BusinessCombinationConsiderationTransferredNotesPayable_a6e9ec39-aa49-449a-88c3-b6e39ca1a6f0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_bd962bc9-b776-46f9-8263-25893adcac75" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_8a5ac7d5-0d7d-49bf-8562-8ed22d1c2bbe" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_bd962bc9-b776-46f9-8263-25893adcac75" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_a547521c-cda5-4d2a-bfa1-070e31bddd22" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_8a5ac7d5-0d7d-49bf-8562-8ed22d1c2bbe" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_a547521c-cda5-4d2a-bfa1-070e31bddd22" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_6e8e2dc9-bb9e-4f64-8c60-8c085071c4f2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2b7cf2d5-e677-49b1-bf42-09307758921a" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_6e8e2dc9-bb9e-4f64-8c60-8c085071c4f2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets_3f0b9014-ad46-4c2c-a4a6-6718e2b1d22c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_6e8e2dc9-bb9e-4f64-8c60-8c085071c4f2" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssets_3f0b9014-ad46-4c2c-a4a6-6718e2b1d22c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLand_07e93f59-e409-4469-9275-e66af797ae34" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLand"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_6e8e2dc9-bb9e-4f64-8c60-8c085071c4f2" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLand_07e93f59-e409-4469-9275-e66af797ae34" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedBuildings_9f823f24-b230-4183-bb2c-2cd6257a56f7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedBuildings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_6e8e2dc9-bb9e-4f64-8c60-8c085071c4f2" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedBuildings_9f823f24-b230-4183-bb2c-2cd6257a56f7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment_775ba7f5-6705-4ba5-8dac-59348a37f15f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_6e8e2dc9-bb9e-4f64-8c60-8c085071c4f2" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedEquipment_775ba7f5-6705-4ba5-8dac-59348a37f15f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_e247b831-f5c2-4469-85d2-c3212d30e4ab" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_6e8e2dc9-bb9e-4f64-8c60-8c085071c4f2" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_e247b831-f5c2-4469-85d2-c3212d30e4ab" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_c3c584cd-b3fc-472e-ad16-cab9abe79106" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_6e8e2dc9-bb9e-4f64-8c60-8c085071c4f2" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_c3c584cd-b3fc-472e-ad16-cab9abe79106" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_b6fd269e-0f4d-43f5-afe7-8745f1a2b43b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNetAbstract_6e8e2dc9-bb9e-4f64-8c60-8c085071c4f2" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_b6fd269e-0f4d-43f5-afe7-8745f1a2b43b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_9836506e-3999-41c4-b07d-77d4ee5725e6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2b7cf2d5-e677-49b1-bf42-09307758921a" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_9836506e-3999-41c4-b07d-77d4ee5725e6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_c07ca93d-f536-418d-8fe3-077da1a787fa" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2b7cf2d5-e677-49b1-bf42-09307758921a" xlink:to="loc_us-gaap_Goodwill_c07ca93d-f536-418d-8fe3-077da1a787fa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_dd453d68-94ca-424c-b41c-7ef15a68ade8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_2b7cf2d5-e677-49b1-bf42-09307758921a" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_dd453d68-94ca-424c-b41c-7ef15a68ade8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_9c00fc47-d79e-4a7f-bc49-062d4d880009" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_dd453d68-94ca-424c-b41c-7ef15a68ade8" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_9c00fc47-d79e-4a7f-bc49-062d4d880009" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9c00fc47-d79e-4a7f-bc49-062d4d880009_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_9c00fc47-d79e-4a7f-bc49-062d4d880009" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_9c00fc47-d79e-4a7f-bc49-062d4d880009_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_cc80efdb-c2d4-4ed5-92ca-a1c7d85a736f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_9c00fc47-d79e-4a7f-bc49-062d4d880009" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_cc80efdb-c2d4-4ed5-92ca-a1c7d85a736f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TransferStationsAcquisitionMember_3d2160f0-a333-4458-9079-30331279911e" xlink:href="cwst-20200630.xsd#cwst_TransferStationsAcquisitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_cc80efdb-c2d4-4ed5-92ca-a1c7d85a736f" xlink:to="loc_cwst_TransferStationsAcquisitionMember_3d2160f0-a333-4458-9079-30331279911e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/BUSINESSCOMBINATIONSScheduleofUnauditedProformaCombinedInformationDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#BUSINESSCOMBINATIONSScheduleofUnauditedProformaCombinedInformationDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/BUSINESSCOMBINATIONSScheduleofUnauditedProformaCombinedInformationDetail" xlink:type="extended" id="ibb6f08986945465e80a1c95e7da538a7_BUSINESSCOMBINATIONSScheduleofUnauditedProformaCombinedInformationDetail"/>
  <link:roleRef roleURI="http://casella.com/role/ACCOUNTSRECEIVABLENETOFALLOWANCEFORCREDITLOSSES" xlink:type="simple" xlink:href="cwst-20200630.xsd#ACCOUNTSRECEIVABLENETOFALLOWANCEFORCREDITLOSSES"/>
  <link:definitionLink xlink:role="http://casella.com/role/ACCOUNTSRECEIVABLENETOFALLOWANCEFORCREDITLOSSES" xlink:type="extended" id="i84b1f2f2ab1c467b8b21a4efde2be1c2_ACCOUNTSRECEIVABLENETOFALLOWANCEFORCREDITLOSSES"/>
  <link:roleRef roleURI="http://casella.com/role/ACCOUNTSRECEIVABLENETOFALLOWANCEFORCREDITLOSSESTables" xlink:type="simple" xlink:href="cwst-20200630.xsd#ACCOUNTSRECEIVABLENETOFALLOWANCEFORCREDITLOSSESTables"/>
  <link:definitionLink xlink:role="http://casella.com/role/ACCOUNTSRECEIVABLENETOFALLOWANCEFORCREDITLOSSESTables" xlink:type="extended" id="i6f0a12b26bf54c158f61613c6b8397d7_ACCOUNTSRECEIVABLENETOFALLOWANCEFORCREDITLOSSESTables"/>
  <link:roleRef roleURI="http://casella.com/role/ACCOUNTSRECEIVABLENETOFALLOWANCEFORCREDITLOSSESDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#ACCOUNTSRECEIVABLENETOFALLOWANCEFORCREDITLOSSESDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/ACCOUNTSRECEIVABLENETOFALLOWANCEFORCREDITLOSSESDetails" xlink:type="extended" id="i803d29f1978a48d3a8f041bb11724508_ACCOUNTSRECEIVABLENETOFALLOWANCEFORCREDITLOSSESDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_b42c3abc-ab43-4fac-b5ce-2e139dfc234f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccountsNotesAndLoansReceivableLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_11563c72-2c32-4136-8e86-5c8714026762" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_b42c3abc-ab43-4fac-b5ce-2e139dfc234f" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_11563c72-2c32-4136-8e86-5c8714026762" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_8cba5cbc-cc22-4ef0-81da-97aa276d8e87" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_11563c72-2c32-4136-8e86-5c8714026762" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_8cba5cbc-cc22-4ef0-81da-97aa276d8e87" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForDoubtfulAccounts_96801c28-c291-4c70-944f-9e71afe7a69b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ProvisionForDoubtfulAccounts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_11563c72-2c32-4136-8e86-5c8714026762" xlink:to="loc_us-gaap_ProvisionForDoubtfulAccounts_96801c28-c291-4c70-944f-9e71afe7a69b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_AccountsReceivableAllowanceForCreditLossWriteoffRecoveries_685374ba-e039-46a2-88ec-6d8b53ad4b25" xlink:href="cwst-20200630.xsd#cwst_AccountsReceivableAllowanceForCreditLossWriteoffRecoveries"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward_11563c72-2c32-4136-8e86-5c8714026762" xlink:to="loc_cwst_AccountsReceivableAllowanceForCreditLossWriteoffRecoveries_685374ba-e039-46a2-88ec-6d8b53ad4b25" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent_2d36e965-5fb1-4b35-beab-cc301545e869" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_735eeac3-b228-4612-9264-590896bb16ba" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccountsNotesAndLoansReceivableLineItems_b42c3abc-ab43-4fac-b5ce-2e139dfc234f" xlink:to="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_735eeac3-b228-4612-9264-590896bb16ba" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_bd57c48e-737e-4803-8870-8f6b4b69a745" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAccountsNotesLoansAndFinancingReceivableTable_735eeac3-b228-4612-9264-590896bb16ba" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_bd57c48e-737e-4803-8870-8f6b4b69a745" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_bd57c48e-737e-4803-8870-8f6b4b69a745_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_bd57c48e-737e-4803-8870-8f6b4b69a745" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_bd57c48e-737e-4803-8870-8f6b4b69a745_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_875e524f-837a-4a19-83d4-a56ee44220f0" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_bd57c48e-737e-4803-8870-8f6b4b69a745" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_875e524f-837a-4a19-83d4-a56ee44220f0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_887f60b3-5c3d-408f-bcfe-82a125821ece" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_875e524f-837a-4a19-83d4-a56ee44220f0" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_887f60b3-5c3d-408f-bcfe-82a125821ece" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/LEASES" xlink:type="simple" xlink:href="cwst-20200630.xsd#LEASES"/>
  <link:definitionLink xlink:role="http://casella.com/role/LEASES" xlink:type="extended" id="i0bdae18f24d440c3894377f472507dd3_LEASES"/>
  <link:roleRef roleURI="http://casella.com/role/LEASESTables" xlink:type="simple" xlink:href="cwst-20200630.xsd#LEASESTables"/>
  <link:definitionLink xlink:role="http://casella.com/role/LEASESTables" xlink:type="extended" id="i9ef4c835b2e546afb9757b28e34bf579_LEASESTables"/>
  <link:roleRef roleURI="http://casella.com/role/LEASESNarrativeDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#LEASESNarrativeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/LEASESNarrativeDetails" xlink:type="extended" id="ia00c8fbc755b4753bef6a9bdbe1fddb4_LEASESNarrativeDetails"/>
  <link:roleRef roleURI="http://casella.com/role/LEASESScheduleofLeaseCostsandOtherLeaseInformationDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#LEASESScheduleofLeaseCostsandOtherLeaseInformationDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/LEASESScheduleofLeaseCostsandOtherLeaseInformationDetails" xlink:type="extended" id="i2b32cf06138049dcb082bbebffe9628d_LEASESScheduleofLeaseCostsandOtherLeaseInformationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_6bdf66c6-d57b-4039-b1ee-5117748cd628" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseCostAbstract_e8530525-1175-4f9f-969b-c718ebe3df68" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LeaseCostAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_6bdf66c6-d57b-4039-b1ee-5117748cd628" xlink:to="loc_us-gaap_LeaseCostAbstract_e8530525-1175-4f9f-969b-c718ebe3df68" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseRightOfUseAssetAmortization_0c98f596-c781-4f77-b5b1-5265eb4d385a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FinanceLeaseRightOfUseAssetAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseCostAbstract_e8530525-1175-4f9f-969b-c718ebe3df68" xlink:to="loc_us-gaap_FinanceLeaseRightOfUseAssetAmortization_0c98f596-c781-4f77-b5b1-5265eb4d385a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseInterestExpense_018bf866-3460-4ee0-8c87-d9085d999cbe" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FinanceLeaseInterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseCostAbstract_e8530525-1175-4f9f-969b-c718ebe3df68" xlink:to="loc_us-gaap_FinanceLeaseInterestExpense_018bf866-3460-4ee0-8c87-d9085d999cbe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseCost_ad801734-4d2a-45ea-868d-a36fa0426c07" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseCostAbstract_e8530525-1175-4f9f-969b-c718ebe3df68" xlink:to="loc_us-gaap_OperatingLeaseCost_ad801734-4d2a-45ea-868d-a36fa0426c07" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermLeaseCost_9a1cc53f-403b-473e-951d-b6c3a8acbf5c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShortTermLeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseCostAbstract_e8530525-1175-4f9f-969b-c718ebe3df68" xlink:to="loc_us-gaap_ShortTermLeaseCost_9a1cc53f-403b-473e-951d-b6c3a8acbf5c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableLeaseCost_723522ea-9ed1-4d99-b12e-64507aad2bdc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_VariableLeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseCostAbstract_e8530525-1175-4f9f-969b-c718ebe3df68" xlink:to="loc_us-gaap_VariableLeaseCost_723522ea-9ed1-4d99-b12e-64507aad2bdc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseCost_4c816f45-b6f3-43fa-9154-7022d780f6c3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseCostAbstract_e8530525-1175-4f9f-969b-c718ebe3df68" xlink:to="loc_us-gaap_LeaseCost_4c816f45-b6f3-43fa-9154-7022d780f6c3" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LeaseOtherInformationAbstract_7a7d870e-143e-42b9-9fb6-c7b3b04f1acf" xlink:href="cwst-20200630.xsd#cwst_LeaseOtherInformationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_6bdf66c6-d57b-4039-b1ee-5117748cd628" xlink:to="loc_cwst_LeaseOtherInformationAbstract_7a7d870e-143e-42b9-9fb6-c7b3b04f1acf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CashFlowOperatingandFinancingActivitiesLesseeAbstract_3a740808-928c-4158-bea7-df7d5cd334a8" xlink:href="cwst-20200630.xsd#cwst_CashFlowOperatingandFinancingActivitiesLesseeAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_LeaseOtherInformationAbstract_7a7d870e-143e-42b9-9fb6-c7b3b04f1acf" xlink:to="loc_cwst_CashFlowOperatingandFinancingActivitiesLesseeAbstract_3a740808-928c-4158-bea7-df7d5cd334a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeasePrincipalPayments_93ef2052-d0ef-49e9-a2aa-16d11fa8388b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FinanceLeasePrincipalPayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_CashFlowOperatingandFinancingActivitiesLesseeAbstract_3a740808-928c-4158-bea7-df7d5cd334a8" xlink:to="loc_us-gaap_FinanceLeasePrincipalPayments_93ef2052-d0ef-49e9-a2aa-16d11fa8388b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeasePayments_5a4078b4-0deb-4b1f-af97-513158edbf67" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeasePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_CashFlowOperatingandFinancingActivitiesLesseeAbstract_3a740808-928c-4158-bea7-df7d5cd334a8" xlink:to="loc_us-gaap_OperatingLeasePayments_5a4078b4-0deb-4b1f-af97-513158edbf67" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability_ec92f29e-e5a5-46d2-84f5-260429904d9c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_LeaseOtherInformationAbstract_7a7d870e-143e-42b9-9fb6-c7b3b04f1acf" xlink:to="loc_us-gaap_RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability_ec92f29e-e5a5-46d2-84f5-260429904d9c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_dfb67373-e120-4b18-acc7-4b2db0711f9b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_LeaseOtherInformationAbstract_7a7d870e-143e-42b9-9fb6-c7b3b04f1acf" xlink:to="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_dfb67373-e120-4b18-acc7-4b2db0711f9b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseWeightedAverageRemainingLeaseTerm1_29b29ca5-6f75-4bfa-aa2d-54bfe3decf85" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FinanceLeaseWeightedAverageRemainingLeaseTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_LeaseOtherInformationAbstract_7a7d870e-143e-42b9-9fb6-c7b3b04f1acf" xlink:to="loc_us-gaap_FinanceLeaseWeightedAverageRemainingLeaseTerm1_29b29ca5-6f75-4bfa-aa2d-54bfe3decf85" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_4a7d91ad-2a99-4bd7-a61c-453e611c3dc5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_LeaseOtherInformationAbstract_7a7d870e-143e-42b9-9fb6-c7b3b04f1acf" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_4a7d91ad-2a99-4bd7-a61c-453e611c3dc5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_47012687-4d6a-4440-9b4a-e8b6b5c5ac2b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_LeaseOtherInformationAbstract_7a7d870e-143e-42b9-9fb6-c7b3b04f1acf" xlink:to="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_47012687-4d6a-4440-9b4a-e8b6b5c5ac2b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_c6b4352d-e938-495e-923a-0f44b3e77d25" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_LeaseOtherInformationAbstract_7a7d870e-143e-42b9-9fb6-c7b3b04f1acf" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_c6b4352d-e938-495e-923a-0f44b3e77d25" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_83598c0e-84c5-4762-bc41-94a07020ceaa" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_6bdf66c6-d57b-4039-b1ee-5117748cd628" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_83598c0e-84c5-4762-bc41-94a07020ceaa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_49989af9-3572-48de-a8f6-18d2344984dc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_83598c0e-84c5-4762-bc41-94a07020ceaa" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_49989af9-3572-48de-a8f6-18d2344984dc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_49989af9-3572-48de-a8f6-18d2344984dc_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_49989af9-3572-48de-a8f6-18d2344984dc" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_49989af9-3572-48de-a8f6-18d2344984dc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_5d7f9000-ede4-43db-94b2-cea2df61b14b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_49989af9-3572-48de-a8f6-18d2344984dc" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_5d7f9000-ede4-43db-94b2-cea2df61b14b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_c40d3402-b0b3-412b-8973-7a3ad820ce51" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_MachineryAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_5d7f9000-ede4-43db-94b2-cea2df61b14b" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_c40d3402-b0b3-412b-8973-7a3ad820ce51" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandfillMember_cbbe245a-d110-4f43-bce0-d371a4139259" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LandfillMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_5d7f9000-ede4-43db-94b2-cea2df61b14b" xlink:to="loc_us-gaap_LandfillMember_cbbe245a-d110-4f43-bce0-d371a4139259" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/LEASESScheduleofEstimatedMinimumFutureLeaseObligationsDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#LEASESScheduleofEstimatedMinimumFutureLeaseObligationsDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/LEASESScheduleofEstimatedMinimumFutureLeaseObligationsDetails" xlink:type="extended" id="ic4b02853099d4dc0b05431dcf18bb417_LEASESScheduleofEstimatedMinimumFutureLeaseObligationsDetails"/>
  <link:roleRef roleURI="http://casella.com/role/GOODWILLANDINTANGIBLEASSETS" xlink:type="simple" xlink:href="cwst-20200630.xsd#GOODWILLANDINTANGIBLEASSETS"/>
  <link:definitionLink xlink:role="http://casella.com/role/GOODWILLANDINTANGIBLEASSETS" xlink:type="extended" id="id3a2e6d760f342eab9da396facebcb20_GOODWILLANDINTANGIBLEASSETS"/>
  <link:roleRef roleURI="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSTables" xlink:type="simple" xlink:href="cwst-20200630.xsd#GOODWILLANDINTANGIBLEASSETSTables"/>
  <link:definitionLink xlink:role="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSTables" xlink:type="extended" id="ibaac3b2b126348e5974a456a081b21b2_GOODWILLANDINTANGIBLEASSETSTables"/>
  <link:roleRef roleURI="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSSummaryofGoodwillActivityDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#GOODWILLANDINTANGIBLEASSETSSummaryofGoodwillActivityDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSSummaryofGoodwillActivityDetail" xlink:type="extended" id="i70e214cc95164c7f8d03e37c97579c81_GOODWILLANDINTANGIBLEASSETSSummaryofGoodwillActivityDetail">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_ec870c29-f6d8-427a-9d60-5cac8299f127" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillRollForward_431f150e-effe-4fec-8ce4-f4d913105b30" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_GoodwillRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_ec870c29-f6d8-427a-9d60-5cac8299f127" xlink:to="loc_us-gaap_GoodwillRollForward_431f150e-effe-4fec-8ce4-f4d913105b30" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_f1e4c140-2076-4460-b3e9-c46aecc895d0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_431f150e-effe-4fec-8ce4-f4d913105b30" xlink:to="loc_us-gaap_Goodwill_f1e4c140-2076-4460-b3e9-c46aecc895d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_74a863c7-7d06-45b2-ac30-c3ad91b7c7fd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_GoodwillAcquiredDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_431f150e-effe-4fec-8ce4-f4d913105b30" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_74a863c7-7d06-45b2-ac30-c3ad91b7c7fd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_a9f0f663-5b09-4003-bafe-9d947d69eb33" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Goodwill"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_4606cb56-f457-4ed7-99ad-4638315e694a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_ec870c29-f6d8-427a-9d60-5cac8299f127" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_4606cb56-f457-4ed7-99ad-4638315e694a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_6afa7b44-27f2-46d1-9275-060c61ad8c71" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_4606cb56-f457-4ed7-99ad-4638315e694a" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_6afa7b44-27f2-46d1-9275-060c61ad8c71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_6afa7b44-27f2-46d1-9275-060c61ad8c71_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_6afa7b44-27f2-46d1-9275-060c61ad8c71" xlink:to="loc_us-gaap_SegmentDomain_6afa7b44-27f2-46d1-9275-060c61ad8c71_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_e97d594d-79ec-4584-adec-0eb81ed8c754" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_6afa7b44-27f2-46d1-9275-060c61ad8c71" xlink:to="loc_us-gaap_SegmentDomain_e97d594d-79ec-4584-adec-0eb81ed8c754" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_EasternRegionMember_2f9dd240-8459-42a1-ad53-a8535e49907a" xlink:href="cwst-20200630.xsd#cwst_EasternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_e97d594d-79ec-4584-adec-0eb81ed8c754" xlink:to="loc_cwst_EasternRegionMember_2f9dd240-8459-42a1-ad53-a8535e49907a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_6f8c4373-2b18-448c-a611-9054d28cc3d9" xlink:href="cwst-20200630.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_e97d594d-79ec-4584-adec-0eb81ed8c754" xlink:to="loc_cwst_WesternRegionMember_6f8c4373-2b18-448c-a611-9054d28cc3d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_addb587d-c996-4da7-a3a3-1beeb2f05a14" xlink:href="cwst-20200630.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_e97d594d-79ec-4584-adec-0eb81ed8c754" xlink:to="loc_cwst_ResourceSolutionsRegionMember_addb587d-c996-4da7-a3a3-1beeb2f05a14" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSSummaryofIntangibleAssetsbyIntangibleAssetTypeDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#GOODWILLANDINTANGIBLEASSETSSummaryofIntangibleAssetsbyIntangibleAssetTypeDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSSummaryofIntangibleAssetsbyIntangibleAssetTypeDetail" xlink:type="extended" id="i2939f7fa15a945dcb5d44440e40acea0_GOODWILLANDINTANGIBLEASSETSSummaryofIntangibleAssetsbyIntangibleAssetTypeDetail">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_805c186a-83a1-4ca6-ba76-d15fd0d09f2f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_7e22159f-9e2f-4f23-92bc-c9bda2805703" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_805c186a-83a1-4ca6-ba76-d15fd0d09f2f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_7e22159f-9e2f-4f23-92bc-c9bda2805703" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_1dc0ad6a-f827-43ab-80bd-1bd333aaed82" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_805c186a-83a1-4ca6-ba76-d15fd0d09f2f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_1dc0ad6a-f827-43ab-80bd-1bd333aaed82" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_fe40520d-d51c-4b26-b5df-e86975e28914" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_805c186a-83a1-4ca6-ba76-d15fd0d09f2f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_fe40520d-d51c-4b26-b5df-e86975e28914" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_05feae2e-8d45-4535-bbc1-c0f8e18c617c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_805c186a-83a1-4ca6-ba76-d15fd0d09f2f" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_05feae2e-8d45-4535-bbc1-c0f8e18c617c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_7ad454a6-4d24-4139-bb2c-90eb65aa4531" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_05feae2e-8d45-4535-bbc1-c0f8e18c617c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_7ad454a6-4d24-4139-bb2c-90eb65aa4531" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_7ad454a6-4d24-4139-bb2c-90eb65aa4531_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_7ad454a6-4d24-4139-bb2c-90eb65aa4531" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_7ad454a6-4d24-4139-bb2c-90eb65aa4531_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_edf9e52e-789c-46cb-9fa2-0676716c224e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_7ad454a6-4d24-4139-bb2c-90eb65aa4531" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_edf9e52e-789c-46cb-9fa2-0676716c224e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_98da055f-750f-4d25-947b-bb4b225b9275" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_edf9e52e-789c-46cb-9fa2-0676716c224e" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_98da055f-750f-4d25-947b-bb4b225b9275" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerListsMember_229e175c-3052-429e-b93b-4df540907e34" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CustomerListsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_edf9e52e-789c-46cb-9fa2-0676716c224e" xlink:to="loc_us-gaap_CustomerListsMember_229e175c-3052-429e-b93b-4df540907e34" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSNarrativeDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#GOODWILLANDINTANGIBLEASSETSNarrativeDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSNarrativeDetail" xlink:type="extended" id="i69708253eeb641a2a246ced3b927a553_GOODWILLANDINTANGIBLEASSETSNarrativeDetail"/>
  <link:roleRef roleURI="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSSummaryofIntangibleAmortizationExpenseEstimatedDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#GOODWILLANDINTANGIBLEASSETSSummaryofIntangibleAmortizationExpenseEstimatedDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/GOODWILLANDINTANGIBLEASSETSSummaryofIntangibleAmortizationExpenseEstimatedDetail" xlink:type="extended" id="ie89805880f8b454b80e66ece7cd417f3_GOODWILLANDINTANGIBLEASSETSSummaryofIntangibleAmortizationExpenseEstimatedDetail"/>
  <link:roleRef roleURI="http://casella.com/role/ACCRUEDFINALCAPPINGCLOSUREANDPOSTCLOSURE" xlink:type="simple" xlink:href="cwst-20200630.xsd#ACCRUEDFINALCAPPINGCLOSUREANDPOSTCLOSURE"/>
  <link:definitionLink xlink:role="http://casella.com/role/ACCRUEDFINALCAPPINGCLOSUREANDPOSTCLOSURE" xlink:type="extended" id="i69c69944dac147fabc1c840edd121d4f_ACCRUEDFINALCAPPINGCLOSUREANDPOSTCLOSURE"/>
  <link:roleRef roleURI="http://casella.com/role/ACCRUEDFINALCAPPINGCLOSUREANDPOSTCLOSURETables" xlink:type="simple" xlink:href="cwst-20200630.xsd#ACCRUEDFINALCAPPINGCLOSUREANDPOSTCLOSURETables"/>
  <link:definitionLink xlink:role="http://casella.com/role/ACCRUEDFINALCAPPINGCLOSUREANDPOSTCLOSURETables" xlink:type="extended" id="i0edea356ec3f46d0967f9b520757c32e_ACCRUEDFINALCAPPINGCLOSUREANDPOSTCLOSURETables"/>
  <link:roleRef roleURI="http://casella.com/role/ACCRUEDFINALCAPPINGCLOSUREANDPOSTCLOSURESummaryofChangestoAccruedCappingClosureandPostClosureLiabilitiesDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#ACCRUEDFINALCAPPINGCLOSUREANDPOSTCLOSURESummaryofChangestoAccruedCappingClosureandPostClosureLiabilitiesDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/ACCRUEDFINALCAPPINGCLOSUREANDPOSTCLOSURESummaryofChangestoAccruedCappingClosureandPostClosureLiabilitiesDetail" xlink:type="extended" id="icb5a9e486d84467c902308b72daf6e01_ACCRUEDFINALCAPPINGCLOSUREANDPOSTCLOSURESummaryofChangestoAccruedCappingClosureandPostClosureLiabilitiesDetail"/>
  <link:roleRef roleURI="http://casella.com/role/DEBT" xlink:type="simple" xlink:href="cwst-20200630.xsd#DEBT"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBT" xlink:type="extended" id="i12f72c954b044abbb80caf644d308f77_DEBT"/>
  <link:roleRef roleURI="http://casella.com/role/DEBTTables" xlink:type="simple" xlink:href="cwst-20200630.xsd#DEBTTables"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTTables" xlink:type="extended" id="i7dde8eca1ee444d0a555b06c2c9d0701_DEBTTables"/>
  <link:roleRef roleURI="http://casella.com/role/DEBTSummaryofComponentsofLongTermDebtandCapitalLeasesbyDebtInstrumentDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#DEBTSummaryofComponentsofLongTermDebtandCapitalLeasesbyDebtInstrumentDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTSummaryofComponentsofLongTermDebtandCapitalLeasesbyDebtInstrumentDetail" xlink:type="extended" id="ibe3763e2afd34d67974eef3a4bd68939_DEBTSummaryofComponentsofLongTermDebtandCapitalLeasesbyDebtInstrumentDetail">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_bd8cfb46-2077-46ff-9619-c56f5d222619" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtAndCapitalLeaseObligations_a77d8b7b-e1fc-4b8c-af12-c0b30dab54de" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bd8cfb46-2077-46ff-9619-c56f5d222619" xlink:to="loc_us-gaap_DebtAndCapitalLeaseObligations_a77d8b7b-e1fc-4b8c-af12-c0b30dab54de" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseLiability_225857c0-beb3-4e8e-9428-88bb576fef02" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FinanceLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bd8cfb46-2077-46ff-9619-c56f5d222619" xlink:to="loc_us-gaap_FinanceLeaseLiability_225857c0-beb3-4e8e-9428-88bb576fef02" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_bcaac256-c5f2-4d3a-aee1-75033191fced" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bd8cfb46-2077-46ff-9619-c56f5d222619" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_bcaac256-c5f2-4d3a-aee1-75033191fced" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_134867fd-539a-40cf-bf7f-1a67808fac16" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bd8cfb46-2077-46ff-9619-c56f5d222619" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsIncludingCurrentMaturities_134867fd-539a-40cf-bf7f-1a67808fac16" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_755f577e-2ad5-4a56-8dee-87fcece34576" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bd8cfb46-2077-46ff-9619-c56f5d222619" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_755f577e-2ad5-4a56-8dee-87fcece34576" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_df616c6a-2b19-4c0a-a9ec-b404743eb8a5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bd8cfb46-2077-46ff-9619-c56f5d222619" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_df616c6a-2b19-4c0a-a9ec-b404743eb8a5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_dd853a20-d143-4a17-a35c-4d31e4eb3d3b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bd8cfb46-2077-46ff-9619-c56f5d222619" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_dd853a20-d143-4a17-a35c-4d31e4eb3d3b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_200e3f5c-1423-49b5-9d35-f4e0000d4d29" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bd8cfb46-2077-46ff-9619-c56f5d222619" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_200e3f5c-1423-49b5-9d35-f4e0000d4d29" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_cc679aea-a5eb-4fd0-a148-f2e9f91122ae" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bd8cfb46-2077-46ff-9619-c56f5d222619" xlink:to="loc_us-gaap_FinanceLeaseWeightedAverageDiscountRatePercent_cc679aea-a5eb-4fd0-a148-f2e9f91122ae" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtWeightedAverageInterestRate_6d6649ee-43de-4061-bebe-669fd2d2fa2a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bd8cfb46-2077-46ff-9619-c56f5d222619" xlink:to="loc_us-gaap_DebtWeightedAverageInterestRate_6d6649ee-43de-4061-bebe-669fd2d2fa2a" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_57df8475-df9a-4717-9327-03654b8c3454" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bd8cfb46-2077-46ff-9619-c56f5d222619" xlink:to="loc_us-gaap_DebtInstrumentTable_57df8475-df9a-4717-9327-03654b8c3454" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_8d92072d-63b1-4070-8108-ced38aed2aca" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_57df8475-df9a-4717-9327-03654b8c3454" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_8d92072d-63b1-4070-8108-ced38aed2aca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_8d92072d-63b1-4070-8108-ced38aed2aca_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_8d92072d-63b1-4070-8108-ced38aed2aca" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_8d92072d-63b1-4070-8108-ced38aed2aca_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_b29912b1-20a8-4186-b5fa-aff163e263d4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_8d92072d-63b1-4070-8108-ced38aed2aca" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_b29912b1-20a8-4186-b5fa-aff163e263d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevolvingCreditFacilityDueMayTwoThousandTwentyThreeMember_46139453-0402-46b3-826a-13ca0572f720" xlink:href="cwst-20200630.xsd#cwst_RevolvingCreditFacilityDueMayTwoThousandTwentyThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b29912b1-20a8-4186-b5fa-aff163e263d4" xlink:to="loc_cwst_RevolvingCreditFacilityDueMayTwoThousandTwentyThreeMember_46139453-0402-46b3-826a-13ca0572f720" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_89c08ab2-edb1-42df-9796-b522afecd9b6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b29912b1-20a8-4186-b5fa-aff163e263d4" xlink:to="loc_us-gaap_SecuredDebtMember_89c08ab2-edb1-42df-9796-b522afecd9b6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CreditFacilityMember_adc0dc68-e370-4626-bbbf-bfe521d7662e" xlink:href="cwst-20200630.xsd#cwst_CreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b29912b1-20a8-4186-b5fa-aff163e263d4" xlink:to="loc_cwst_CreditFacilityMember_adc0dc68-e370-4626-bbbf-bfe521d7662e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondsSeriesTwoThousandFourteenDueTwoThousandFortyFourMember_44f1db2c-899a-4aa8-be89-1227f34d4f9c" xlink:href="cwst-20200630.xsd#cwst_RevenueBondsSeriesTwoThousandFourteenDueTwoThousandFortyFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b29912b1-20a8-4186-b5fa-aff163e263d4" xlink:to="loc_cwst_RevenueBondsSeriesTwoThousandFourteenDueTwoThousandFortyFourMember_44f1db2c-899a-4aa8-be89-1227f34d4f9c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember_8372364b-78b3-4e70-9194-3fe10013258f" xlink:href="cwst-20200630.xsd#cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b29912b1-20a8-4186-b5fa-aff163e263d4" xlink:to="loc_cwst_RevenueBondSeriesTwoThousandFourteenRTwoDueTwoThousandFortyFourMember_8372364b-78b3-4e70-9194-3fe10013258f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FinanceAuthorityofMaineSolidWasteDisposalRevenueBondsSeries2005R3FAMEBonds2005R3Member_47944c5c-2d09-4c88-b9d5-dc3f6bcef5b2" xlink:href="cwst-20200630.xsd#cwst_FinanceAuthorityofMaineSolidWasteDisposalRevenueBondsSeries2005R3FAMEBonds2005R3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b29912b1-20a8-4186-b5fa-aff163e263d4" xlink:to="loc_cwst_FinanceAuthorityofMaineSolidWasteDisposalRevenueBondsSeries2005R3FAMEBonds2005R3Member_47944c5c-2d09-4c88-b9d5-dc3f6bcef5b2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R1Member_7eeebe7e-9501-4043-b141-d4d416d2abad" xlink:href="cwst-20200630.xsd#cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b29912b1-20a8-4186-b5fa-aff163e263d4" xlink:to="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R1Member_7eeebe7e-9501-4043-b141-d4d416d2abad" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R2Member_7f3f7d8e-637e-42b7-a562-387cc1a7a1bf" xlink:href="cwst-20200630.xsd#cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b29912b1-20a8-4186-b5fa-aff163e263d4" xlink:to="loc_cwst_FinanceAuthorityOfMaineSolidWasteDisposalRevenueBondsSeries2015R2Member_7f3f7d8e-637e-42b7-a562-387cc1a7a1bf" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_LongTermRevenueBondSeriesTwoThousandThirteenDueTwoThousandThirtySixMember_259c3ace-5b45-4ac4-a818-ee7d5b3cfb74" xlink:href="cwst-20200630.xsd#cwst_LongTermRevenueBondSeriesTwoThousandThirteenDueTwoThousandThirtySixMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b29912b1-20a8-4186-b5fa-aff163e263d4" xlink:to="loc_cwst_LongTermRevenueBondSeriesTwoThousandThirteenDueTwoThousandThirtySixMember_259c3ace-5b45-4ac4-a818-ee7d5b3cfb74" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevenueBondSeriesTwoThousandThirteenDueTwoThousandTwentyNineMember_37a4a223-6bba-4614-806f-2439f7adcb60" xlink:href="cwst-20200630.xsd#cwst_RevenueBondSeriesTwoThousandThirteenDueTwoThousandTwentyNineMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b29912b1-20a8-4186-b5fa-aff163e263d4" xlink:to="loc_cwst_RevenueBondSeriesTwoThousandThirteenDueTwoThousandTwentyNineMember_37a4a223-6bba-4614-806f-2439f7adcb60" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NotesPayableMember_d382eb4a-16a4-4ffc-8add-e0969409d2fb" xlink:href="cwst-20200630.xsd#cwst_NotesPayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b29912b1-20a8-4186-b5fa-aff163e263d4" xlink:to="loc_cwst_NotesPayableMember_d382eb4a-16a4-4ffc-8add-e0969409d2fb" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_0bfd5e5f-51cc-48fd-b26c-57da832c306f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_57df8475-df9a-4717-9327-03654b8c3454" xlink:to="loc_us-gaap_DebtInstrumentAxis_0bfd5e5f-51cc-48fd-b26c-57da832c306f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_0bfd5e5f-51cc-48fd-b26c-57da832c306f_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_0bfd5e5f-51cc-48fd-b26c-57da832c306f" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_0bfd5e5f-51cc-48fd-b26c-57da832c306f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_c8c1b781-e210-4d9a-8585-b9ac951da161" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_0bfd5e5f-51cc-48fd-b26c-57da832c306f" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_c8c1b781-e210-4d9a-8585-b9ac951da161" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TermFacilityDueMay2023TermLoanBFacilityMember_28a98ada-b0d2-4f9a-8b98-07bc0f2ccc07" xlink:href="cwst-20200630.xsd#cwst_TermFacilityDueMay2023TermLoanBFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c8c1b781-e210-4d9a-8585-b9ac951da161" xlink:to="loc_cwst_TermFacilityDueMay2023TermLoanBFacilityMember_28a98ada-b0d2-4f9a-8b98-07bc0f2ccc07" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_229a133c-394f-4045-b266-463a2f06fe7e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_57df8475-df9a-4717-9327-03654b8c3454" xlink:to="loc_us-gaap_VariableRateAxis_229a133c-394f-4045-b266-463a2f06fe7e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_229a133c-394f-4045-b266-463a2f06fe7e_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_229a133c-394f-4045-b266-463a2f06fe7e" xlink:to="loc_us-gaap_VariableRateDomain_229a133c-394f-4045-b266-463a2f06fe7e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_291e043a-05ff-4c39-8ad8-7f2d04e4cd06" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_229a133c-394f-4045-b266-463a2f06fe7e" xlink:to="loc_us-gaap_VariableRateDomain_291e043a-05ff-4c39-8ad8-7f2d04e4cd06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_e9b37cb2-a1af-4744-ab5e-eda477ac0b7b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LondonInterbankOfferedRateLIBORMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_291e043a-05ff-4c39-8ad8-7f2d04e4cd06" xlink:to="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_e9b37cb2-a1af-4744-ab5e-eda477ac0b7b" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTCreditFacilityDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#DEBTCreditFacilityDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTCreditFacilityDetails" xlink:type="extended" id="i3f57a6aa63b6429897224bfc399d4162_DEBTCreditFacilityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_ae9beaf5-408f-4592-adfc-71e5a076fe6f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_a40db764-d57f-471b-ac76-19aaea123ca3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9beaf5-408f-4592-adfc-71e5a076fe6f" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_a40db764-d57f-471b-ac76-19aaea123ca3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_d6641d66-4c9b-4ef9-b970-6517949d4847" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9beaf5-408f-4592-adfc-71e5a076fe6f" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_d6641d66-4c9b-4ef9-b970-6517949d4847" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_AdditionalRevolverCapacity_70a5670f-d630-4cd6-ae45-de55f8d4efbf" xlink:href="cwst-20200630.xsd#cwst_AdditionalRevolverCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9beaf5-408f-4592-adfc-71e5a076fe6f" xlink:to="loc_cwst_AdditionalRevolverCapacity_70a5670f-d630-4cd6-ae45-de55f8d4efbf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_3f506021-bf15-4248-8a50-2108e94d1d21" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9beaf5-408f-4592-adfc-71e5a076fe6f" xlink:to="loc_us-gaap_DebtInstrumentTerm_3f506021-bf15-4248-8a50-2108e94d1d21" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeBasisSpreadOnVariableRate_c7090b7b-dab2-4e9d-be67-014a5c0b80ba" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeBasisSpreadOnVariableRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9beaf5-408f-4592-adfc-71e5a076fe6f" xlink:to="loc_us-gaap_DerivativeBasisSpreadOnVariableRate_c7090b7b-dab2-4e9d-be67-014a5c0b80ba" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_DebtInstrumentCovenantTermsNetLeverageRatio_1b00ce13-fa33-4407-b465-2cd77ee51845" xlink:href="cwst-20200630.xsd#cwst_DebtInstrumentCovenantTermsNetLeverageRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9beaf5-408f-4592-adfc-71e5a076fe6f" xlink:to="loc_cwst_DebtInstrumentCovenantTermsNetLeverageRatio_1b00ce13-fa33-4407-b465-2cd77ee51845" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_6ff8c64a-ed5e-4720-9801-69b9a4391981" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9beaf5-408f-4592-adfc-71e5a076fe6f" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_6ff8c64a-ed5e-4720-9801-69b9a4391981" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_0e2ad1ff-04cc-4054-ae69-ec7a4778cda5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9beaf5-408f-4592-adfc-71e5a076fe6f" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_0e2ad1ff-04cc-4054-ae69-ec7a4778cda5" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_30989450-d39a-4f2a-a32a-d06cf4c53d90" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_ae9beaf5-408f-4592-adfc-71e5a076fe6f" xlink:to="loc_us-gaap_DebtInstrumentTable_30989450-d39a-4f2a-a32a-d06cf4c53d90" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_3e997165-85f0-47bb-a3a7-fe4518178c45" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_30989450-d39a-4f2a-a32a-d06cf4c53d90" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_3e997165-85f0-47bb-a3a7-fe4518178c45" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_3e997165-85f0-47bb-a3a7-fe4518178c45_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_3e997165-85f0-47bb-a3a7-fe4518178c45" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_3e997165-85f0-47bb-a3a7-fe4518178c45_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_223ec562-3ded-4f77-95f2-989461c5252e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_3e997165-85f0-47bb-a3a7-fe4518178c45" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_223ec562-3ded-4f77-95f2-989461c5252e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_dcaaa749-cb6a-4ed0-8d8f-8f905455b5fe" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_223ec562-3ded-4f77-95f2-989461c5252e" xlink:to="loc_us-gaap_SecuredDebtMember_dcaaa749-cb6a-4ed0-8d8f-8f905455b5fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_a00d6c4f-4220-49c2-8f84-e2b20a116e5c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_223ec562-3ded-4f77-95f2-989461c5252e" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_a00d6c4f-4220-49c2-8f84-e2b20a116e5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SeniorSecuredAssetBasedRevolvingCreditFacilityDueTwoThousandAndTwentyThreeMember_875408d3-8136-4369-88e4-f58ff2222c9a" xlink:href="cwst-20200630.xsd#cwst_SeniorSecuredAssetBasedRevolvingCreditFacilityDueTwoThousandAndTwentyThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_223ec562-3ded-4f77-95f2-989461c5252e" xlink:to="loc_cwst_SeniorSecuredAssetBasedRevolvingCreditFacilityDueTwoThousandAndTwentyThreeMember_875408d3-8136-4369-88e4-f58ff2222c9a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_ef01ce9d-d3ff-4359-bde3-886a2ba70006" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_30989450-d39a-4f2a-a32a-d06cf4c53d90" xlink:to="loc_us-gaap_DebtInstrumentAxis_ef01ce9d-d3ff-4359-bde3-886a2ba70006" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_ef01ce9d-d3ff-4359-bde3-886a2ba70006_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_ef01ce9d-d3ff-4359-bde3-886a2ba70006" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_ef01ce9d-d3ff-4359-bde3-886a2ba70006_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_860dfff3-f051-4252-aa22-9b550fc2c895" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_ef01ce9d-d3ff-4359-bde3-886a2ba70006" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_860dfff3-f051-4252-aa22-9b550fc2c895" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TermFacilitydue2023TermLoanAFacilityMember_8dcd18a8-3964-4e74-bf1c-4c0c45465a26" xlink:href="cwst-20200630.xsd#cwst_TermFacilitydue2023TermLoanAFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_860dfff3-f051-4252-aa22-9b550fc2c895" xlink:to="loc_cwst_TermFacilitydue2023TermLoanAFacilityMember_8dcd18a8-3964-4e74-bf1c-4c0c45465a26" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RevolvingCreditFacilityDue2023Member_be801143-cbd0-4b3a-947e-a7a892547ff1" xlink:href="cwst-20200630.xsd#cwst_RevolvingCreditFacilityDue2023Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_860dfff3-f051-4252-aa22-9b550fc2c895" xlink:to="loc_cwst_RevolvingCreditFacilityDue2023Member_be801143-cbd0-4b3a-947e-a7a892547ff1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_15cff60c-3d69-4555-839b-a6318fbfc530" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_30989450-d39a-4f2a-a32a-d06cf4c53d90" xlink:to="loc_us-gaap_CreditFacilityAxis_15cff60c-3d69-4555-839b-a6318fbfc530" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_15cff60c-3d69-4555-839b-a6318fbfc530_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_15cff60c-3d69-4555-839b-a6318fbfc530" xlink:to="loc_us-gaap_CreditFacilityDomain_15cff60c-3d69-4555-839b-a6318fbfc530_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_f0f2e916-d89a-46e2-837a-cafd5dd20982" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_15cff60c-3d69-4555-839b-a6318fbfc530" xlink:to="loc_us-gaap_CreditFacilityDomain_f0f2e916-d89a-46e2-837a-cafd5dd20982" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_ffa03e43-e208-4b60-b607-faa23e45d4b2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_f0f2e916-d89a-46e2-837a-cafd5dd20982" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_ffa03e43-e208-4b60-b607-faa23e45d4b2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_86b90ca4-a5a9-4807-9876-a0225c9d650e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_f0f2e916-d89a-46e2-837a-cafd5dd20982" xlink:to="loc_us-gaap_LineOfCreditMember_86b90ca4-a5a9-4807-9876-a0225c9d650e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_61daacbe-e462-4614-8ea0-7b456086325b" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_30989450-d39a-4f2a-a32a-d06cf4c53d90" xlink:to="loc_srt_RangeAxis_61daacbe-e462-4614-8ea0-7b456086325b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_61daacbe-e462-4614-8ea0-7b456086325b_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_61daacbe-e462-4614-8ea0-7b456086325b" xlink:to="loc_srt_RangeMember_61daacbe-e462-4614-8ea0-7b456086325b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_d068e078-28ca-428d-a131-26e5d76b6ab0" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_61daacbe-e462-4614-8ea0-7b456086325b" xlink:to="loc_srt_RangeMember_d068e078-28ca-428d-a131-26e5d76b6ab0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_5d6db184-3e33-48d6-ad81-dab52df6f5f0" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_d068e078-28ca-428d-a131-26e5d76b6ab0" xlink:to="loc_srt_MaximumMember_5d6db184-3e33-48d6-ad81-dab52df6f5f0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_410b387b-fa2e-44f4-a851-dc3b4c8fd7db" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_30989450-d39a-4f2a-a32a-d06cf4c53d90" xlink:to="loc_us-gaap_VariableRateAxis_410b387b-fa2e-44f4-a851-dc3b4c8fd7db" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_410b387b-fa2e-44f4-a851-dc3b4c8fd7db_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_410b387b-fa2e-44f4-a851-dc3b4c8fd7db" xlink:to="loc_us-gaap_VariableRateDomain_410b387b-fa2e-44f4-a851-dc3b4c8fd7db_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_14f6d11e-68d6-4b88-9474-fde84db731ea" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_410b387b-fa2e-44f4-a851-dc3b4c8fd7db" xlink:to="loc_us-gaap_VariableRateDomain_14f6d11e-68d6-4b88-9474-fde84db731ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_bd0d6096-7cf0-40bc-a990-cb28f59a11da" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LondonInterbankOfferedRateLIBORMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_14f6d11e-68d6-4b88-9474-fde84db731ea" xlink:to="loc_us-gaap_LondonInterbankOfferedRateLIBORMember_bd0d6096-7cf0-40bc-a990-cb28f59a11da" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTCashFlowHedgesDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#DEBTCashFlowHedgesDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTCashFlowHedgesDetails" xlink:type="extended" id="if0c29ba2c1ae4b3dbe2eae17c488a32c_DEBTCashFlowHedgesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_61a53056-313b-4168-ac86-5268666a687a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfInterestRateDerivativesHeld_f7d38165-bdec-4088-94a4-5c4ac2975749" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NumberOfInterestRateDerivativesHeld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_61a53056-313b-4168-ac86-5268666a687a" xlink:to="loc_us-gaap_NumberOfInterestRateDerivativesHeld_f7d38165-bdec-4088-94a4-5c4ac2975749" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeNotionalAmount_246920bb-1872-4534-bb41-a5b66de79bf2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeNotionalAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_61a53056-313b-4168-ac86-5268666a687a" xlink:to="loc_us-gaap_DerivativeNotionalAmount_246920bb-1872-4534-bb41-a5b66de79bf2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeFloorInterestRate_41fb4311-52e9-4d95-bc78-ed11b15502ce" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeFloorInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_61a53056-313b-4168-ac86-5268666a687a" xlink:to="loc_us-gaap_DerivativeFloorInterestRate_41fb4311-52e9-4d95-bc78-ed11b15502ce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax_f4af37ce-3662-426e-b46f-44f30c646626" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_61a53056-313b-4168-ac86-5268666a687a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossBeforeReclassificationAndTax_f4af37ce-3662-426e-b46f-44f30c646626" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromHedgeInvestingActivities_e629cf9d-c7d1-437e-bb96-2e3a22f45d2b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ProceedsFromHedgeInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_61a53056-313b-4168-ac86-5268666a687a" xlink:to="loc_us-gaap_ProceedsFromHedgeInvestingActivities_e629cf9d-c7d1-437e-bb96-2e3a22f45d2b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeAverageFixedInterestRate_05e5ee1b-ed50-4eb8-b0f1-7229fe8888bc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeAverageFixedInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_61a53056-313b-4168-ac86-5268666a687a" xlink:to="loc_us-gaap_DerivativeAverageFixedInterestRate_05e5ee1b-ed50-4eb8-b0f1-7229fe8888bc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_f637ec85-1bad-404a-8e83-d693f5906c1e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_61a53056-313b-4168-ac86-5268666a687a" xlink:to="loc_us-gaap_DebtInstrumentTable_f637ec85-1bad-404a-8e83-d693f5906c1e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_c72554c5-38ed-4aff-9032-c3f3e4dde6a0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_f637ec85-1bad-404a-8e83-d693f5906c1e" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_c72554c5-38ed-4aff-9032-c3f3e4dde6a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_c72554c5-38ed-4aff-9032-c3f3e4dde6a0_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_c72554c5-38ed-4aff-9032-c3f3e4dde6a0" xlink:to="loc_us-gaap_HedgingRelationshipDomain_c72554c5-38ed-4aff-9032-c3f3e4dde6a0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingRelationshipDomain_77c59cba-5a57-4090-8b53-236e8ce556a0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_HedgingRelationshipDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis_c72554c5-38ed-4aff-9032-c3f3e4dde6a0" xlink:to="loc_us-gaap_HedgingRelationshipDomain_77c59cba-5a57-4090-8b53-236e8ce556a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashFlowHedgingMember_64f192f1-8074-48f2-87c8-0e5337ca515e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CashFlowHedgingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingRelationshipDomain_77c59cba-5a57-4090-8b53-236e8ce556a0" xlink:to="loc_us-gaap_CashFlowHedgingMember_64f192f1-8074-48f2-87c8-0e5337ca515e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_ab5f8794-025b-46c5-846a-cc4512a9600f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_f637ec85-1bad-404a-8e83-d693f5906c1e" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_ab5f8794-025b-46c5-846a-cc4512a9600f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_ab5f8794-025b-46c5-846a-cc4512a9600f_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_ab5f8794-025b-46c5-846a-cc4512a9600f" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_ab5f8794-025b-46c5-846a-cc4512a9600f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_72e9f2df-65b9-40ec-a52d-c05801aac7b0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_ab5f8794-025b-46c5-846a-cc4512a9600f" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_72e9f2df-65b9-40ec-a52d-c05801aac7b0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateContractMember_be1b2e05-4833-4cd8-a5b4-0873eeb43686" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InterestRateContractMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_72e9f2df-65b9-40ec-a52d-c05801aac7b0" xlink:to="loc_us-gaap_InterestRateContractMember_be1b2e05-4833-4cd8-a5b4-0873eeb43686" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ForwardInterestRateContractMember_31d7b83d-f8d9-44ac-96e3-3aa71b92b65f" xlink:href="cwst-20200630.xsd#cwst_ForwardInterestRateContractMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_72e9f2df-65b9-40ec-a52d-c05801aac7b0" xlink:to="loc_cwst_ForwardInterestRateContractMember_31d7b83d-f8d9-44ac-96e3-3aa71b92b65f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementScenarioAxis_f8983876-b27b-48d0-812a-b4f71328194a" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_StatementScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_f637ec85-1bad-404a-8e83-d693f5906c1e" xlink:to="loc_srt_StatementScenarioAxis_f8983876-b27b-48d0-812a-b4f71328194a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_f8983876-b27b-48d0-812a-b4f71328194a_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_f8983876-b27b-48d0-812a-b4f71328194a" xlink:to="loc_srt_ScenarioUnspecifiedDomain_f8983876-b27b-48d0-812a-b4f71328194a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioUnspecifiedDomain_eecacbe0-9e5d-4825-809f-49bcbb21221d" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_ScenarioUnspecifiedDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_f8983876-b27b-48d0-812a-b4f71328194a" xlink:to="loc_srt_ScenarioUnspecifiedDomain_eecacbe0-9e5d-4825-809f-49bcbb21221d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScenarioForecastMember_5a5a39ff-52de-4b41-b546-4e58f70a0da7" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_ScenarioForecastMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ScenarioUnspecifiedDomain_eecacbe0-9e5d-4825-809f-49bcbb21221d" xlink:to="loc_srt_ScenarioForecastMember_5a5a39ff-52de-4b41-b546-4e58f70a0da7" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTScheduleOfCashFlowHedgesRelatedtoDerivativeInstrumentsOntheBalanceSheetDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#DEBTScheduleOfCashFlowHedgesRelatedtoDerivativeInstrumentsOntheBalanceSheetDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTScheduleOfCashFlowHedgesRelatedtoDerivativeInstrumentsOntheBalanceSheetDetails" xlink:type="extended" id="i3af660ee18a24d3a85e70b4ccf95b2c8_DEBTScheduleOfCashFlowHedgesRelatedtoDerivativeInstrumentsOntheBalanceSheetDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativesFairValueLineItems_2d353570-c4c4-4d51-88ce-3c34b3eecb5c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativesFairValueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLiabilityFairValueGrossAsset_f15ff5d9-e319-4c85-b0ae-9856e91c9716" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeLiabilityFairValueGrossAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_2d353570-c4c4-4d51-88ce-3c34b3eecb5c" xlink:to="loc_us-gaap_DerivativeLiabilityFairValueGrossAsset_f15ff5d9-e319-4c85-b0ae-9856e91c9716" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossFinancialLiabilityFairValueOptionUnrealizedGainLossArisingDuringPeriodBeforeTax_167cacad-f7c9-40da-8ed8-6e66daa2ceb1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossFinancialLiabilityFairValueOptionUnrealizedGainLossArisingDuringPeriodBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_2d353570-c4c4-4d51-88ce-3c34b3eecb5c" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossFinancialLiabilityFairValueOptionUnrealizedGainLossArisingDuringPeriodBeforeTax_167cacad-f7c9-40da-8ed8-6e66daa2ceb1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossFinancialLiabilityFairValueOptionUnrealizedGainLossArisingDuringPeriodTax_1a42ad9b-608f-443e-9471-88cde82649f7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossFinancialLiabilityFairValueOptionUnrealizedGainLossArisingDuringPeriodTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_2d353570-c4c4-4d51-88ce-3c34b3eecb5c" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossFinancialLiabilityFairValueOptionUnrealizedGainLossArisingDuringPeriodTax_1a42ad9b-608f-443e-9471-88cde82649f7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossFinancialLiabilityFairValueOptionUnrealizedGainLossArisingDuringPeriodAfterTax_3df97e3f-ef65-4197-891f-0f89a1c9f6fc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossFinancialLiabilityFairValueOptionUnrealizedGainLossArisingDuringPeriodAfterTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_2d353570-c4c4-4d51-88ce-3c34b3eecb5c" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossFinancialLiabilityFairValueOptionUnrealizedGainLossArisingDuringPeriodAfterTax_3df97e3f-ef65-4197-891f-0f89a1c9f6fc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_bf065c90-6387-49e3-b0bc-288563dab254" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativesFairValueLineItems_2d353570-c4c4-4d51-88ce-3c34b3eecb5c" xlink:to="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_bf065c90-6387-49e3-b0bc-288563dab254" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_a1b3f8f9-4897-4f78-8f03-8785ca3bd265" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_bf065c90-6387-49e3-b0bc-288563dab254" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_a1b3f8f9-4897-4f78-8f03-8785ca3bd265" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_a1b3f8f9-4897-4f78-8f03-8785ca3bd265_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_a1b3f8f9-4897-4f78-8f03-8785ca3bd265" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_a1b3f8f9-4897-4f78-8f03-8785ca3bd265_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_4f452dd1-240d-4c6a-8d0f-5037c8f978e3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_a1b3f8f9-4897-4f78-8f03-8785ca3bd265" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_4f452dd1-240d-4c6a-8d0f-5037c8f978e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_OtherAccruedLiabilitiesMember_6097e319-bc41-42f0-904b-4037d17ccc47" xlink:href="cwst-20200630.xsd#cwst_OtherAccruedLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_4f452dd1-240d-4c6a-8d0f-5037c8f978e3" xlink:to="loc_cwst_OtherAccruedLiabilitiesMember_6097e319-bc41-42f0-904b-4037d17ccc47" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_2856b63c-6506-4a6e-bdc6-8aeea47b3ccb" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherNoncurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_4f452dd1-240d-4c6a-8d0f-5037c8f978e3" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_2856b63c-6506-4a6e-bdc6-8aeea47b3ccb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_e6f16e72-dcb7-402f-8f3c-35c1570cb8ae" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_4f452dd1-240d-4c6a-8d0f-5037c8f978e3" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_e6f16e72-dcb7-402f-8f3c-35c1570cb8ae" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_73879268-40e2-4977-b9d9-eeac8b867d16" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_bf065c90-6387-49e3-b0bc-288563dab254" xlink:to="loc_us-gaap_HedgingDesignationAxis_73879268-40e2-4977-b9d9-eeac8b867d16" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_73879268-40e2-4977-b9d9-eeac8b867d16_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_73879268-40e2-4977-b9d9-eeac8b867d16" xlink:to="loc_us-gaap_HedgingDesignationDomain_73879268-40e2-4977-b9d9-eeac8b867d16_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_94777645-c2eb-4fa5-af7f-a37e6fe4faa0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_73879268-40e2-4977-b9d9-eeac8b867d16" xlink:to="loc_us-gaap_HedgingDesignationDomain_94777645-c2eb-4fa5-af7f-a37e6fe4faa0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_770b1bf4-a708-4eb8-b11b-e60a3829f9e8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_94777645-c2eb-4fa5-af7f-a37e6fe4faa0" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_770b1bf4-a708-4eb8-b11b-e60a3829f9e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_2ccb2c77-a1c0-4488-8bc6-d05300a2aef5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValuesDerivativesBalanceSheetLocationByDerivativeContractTypeByHedgingDesignationTable_bf065c90-6387-49e3-b0bc-288563dab254" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_2ccb2c77-a1c0-4488-8bc6-d05300a2aef5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_2ccb2c77-a1c0-4488-8bc6-d05300a2aef5_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_2ccb2c77-a1c0-4488-8bc6-d05300a2aef5" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_2ccb2c77-a1c0-4488-8bc6-d05300a2aef5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_9f3c7a4b-faaf-4e04-a27a-1e64e665edad" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_2ccb2c77-a1c0-4488-8bc6-d05300a2aef5" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_9f3c7a4b-faaf-4e04-a27a-1e64e665edad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_43700b31-e42d-452f-9f64-b0879c01eabc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_9f3c7a4b-faaf-4e04-a27a-1e64e665edad" xlink:to="loc_us-gaap_InterestRateSwapMember_43700b31-e42d-452f-9f64-b0879c01eabc" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/DEBTScheduleofLossOnCashFlowHedgesDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#DEBTScheduleofLossOnCashFlowHedgesDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/DEBTScheduleofLossOnCashFlowHedgesDetails" xlink:type="extended" id="idedfa185f86c4616a447d73f594d7966_DEBTScheduleofLossOnCashFlowHedgesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_200b1fcb-48d4-488a-92e3-cae1dc09d6c4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax_d0fbe969-4a85-4b81-897b-07a60e5fcb1b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_200b1fcb-48d4-488a-92e3-cae1dc09d6c4" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossReclassificationBeforeTax_d0fbe969-4a85-4b81-897b-07a60e5fcb1b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_8b9f74ce-3e2e-4386-a3c0-5aec9541d980" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossLineItems_200b1fcb-48d4-488a-92e3-cae1dc09d6c4" xlink:to="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_8b9f74ce-3e2e-4386-a3c0-5aec9541d980" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationAxis_1205baf1-1e28-4250-9e2a-675da8ac38c3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_HedgingDesignationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_8b9f74ce-3e2e-4386-a3c0-5aec9541d980" xlink:to="loc_us-gaap_HedgingDesignationAxis_1205baf1-1e28-4250-9e2a-675da8ac38c3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_1205baf1-1e28-4250-9e2a-675da8ac38c3_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_HedgingDesignationAxis_1205baf1-1e28-4250-9e2a-675da8ac38c3" xlink:to="loc_us-gaap_HedgingDesignationDomain_1205baf1-1e28-4250-9e2a-675da8ac38c3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HedgingDesignationDomain_ed2c2074-8cb1-45c4-9d1d-3578f2ae1f2b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_HedgingDesignationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_HedgingDesignationAxis_1205baf1-1e28-4250-9e2a-675da8ac38c3" xlink:to="loc_us-gaap_HedgingDesignationDomain_ed2c2074-8cb1-45c4-9d1d-3578f2ae1f2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DesignatedAsHedgingInstrumentMember_940ac052-36ca-4a18-8baf-8c374c1735d5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DesignatedAsHedgingInstrumentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_HedgingDesignationDomain_ed2c2074-8cb1-45c4-9d1d-3578f2ae1f2b" xlink:to="loc_us-gaap_DesignatedAsHedgingInstrumentMember_940ac052-36ca-4a18-8baf-8c374c1735d5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_e0377c91-e877-4610-836f-12282f31bbbb" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_8b9f74ce-3e2e-4386-a3c0-5aec9541d980" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_e0377c91-e877-4610-836f-12282f31bbbb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_e0377c91-e877-4610-836f-12282f31bbbb_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_e0377c91-e877-4610-836f-12282f31bbbb" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_e0377c91-e877-4610-836f-12282f31bbbb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_9a18bbd5-bdc1-4405-acfc-a6ee6f4a4f8f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_e0377c91-e877-4610-836f-12282f31bbbb" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_9a18bbd5-bdc1-4405-acfc-a6ee6f4a4f8f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_e0f67d29-e74d-4f5d-b0e2-cc22843816ed" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_9a18bbd5-bdc1-4405-acfc-a6ee6f4a4f8f" xlink:to="loc_us-gaap_InterestRateSwapMember_e0f67d29-e74d-4f5d-b0e2-cc22843816ed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_8ef7bf6b-a84a-4448-bb06-ecb33136561b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipByIncomeStatementLocationByDerivativeInstrumentRiskTable_8b9f74ce-3e2e-4386-a3c0-5aec9541d980" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_8ef7bf6b-a84a-4448-bb06-ecb33136561b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_8ef7bf6b-a84a-4448-bb06-ecb33136561b_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_8ef7bf6b-a84a-4448-bb06-ecb33136561b" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_8ef7bf6b-a84a-4448-bb06-ecb33136561b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_d11e02e4-ae47-4675-a528-4b2b261d9914" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_8ef7bf6b-a84a-4448-bb06-ecb33136561b" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_d11e02e4-ae47-4675-a528-4b2b261d9914" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseMember_d3cb8da3-050e-4a9b-b547-2feb22bd9499" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InterestExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_d11e02e4-ae47-4675-a528-4b2b261d9914" xlink:to="loc_us-gaap_InterestExpenseMember_d3cb8da3-050e-4a9b-b547-2feb22bd9499" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/COMMITMENTSANDCONTINGENCIES" xlink:type="simple" xlink:href="cwst-20200630.xsd#COMMITMENTSANDCONTINGENCIES"/>
  <link:definitionLink xlink:role="http://casella.com/role/COMMITMENTSANDCONTINGENCIES" xlink:type="extended" id="i6e01017b625a4159958f5b534eada301_COMMITMENTSANDCONTINGENCIES"/>
  <link:roleRef roleURI="http://casella.com/role/COMMITMENTSANDCONTINGENCIESTables" xlink:type="simple" xlink:href="cwst-20200630.xsd#COMMITMENTSANDCONTINGENCIESTables"/>
  <link:definitionLink xlink:role="http://casella.com/role/COMMITMENTSANDCONTINGENCIESTables" xlink:type="extended" id="i5c593eeb0c3b4e0bbdf7dfcadd8bfc66_COMMITMENTSANDCONTINGENCIESTables"/>
  <link:roleRef roleURI="http://casella.com/role/COMMITMENTSANDCONTINGENCIESNarrativeDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#COMMITMENTSANDCONTINGENCIESNarrativeDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/COMMITMENTSANDCONTINGENCIESNarrativeDetail" xlink:type="extended" id="i6f080f8309954f2e8cf5a9cbead6afd4_COMMITMENTSANDCONTINGENCIESNarrativeDetail">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_87183105-c160-4b0a-bc51-4405bee29a12" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyEstimateOfPossibleLoss_f960fd60-7c6f-4a42-a890-82c72a0ee241" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LossContingencyEstimateOfPossibleLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_87183105-c160-4b0a-bc51-4405bee29a12" xlink:to="loc_us-gaap_LossContingencyEstimateOfPossibleLoss_f960fd60-7c6f-4a42-a890-82c72a0ee241" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTerm_86b35375-d1f1-4527-812c-a925cd905f21" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_87183105-c160-4b0a-bc51-4405bee29a12" xlink:to="loc_us-gaap_DebtInstrumentTerm_86b35375-d1f1-4527-812c-a925cd905f21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalExitCostsAnticipatedCost_295d9acc-f7f9-4a90-9110-50299f6f2a84" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EnvironmentalExitCostsAnticipatedCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_87183105-c160-4b0a-bc51-4405bee29a12" xlink:to="loc_us-gaap_EnvironmentalExitCostsAnticipatedCost_295d9acc-f7f9-4a90-9110-50299f6f2a84" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingenciesDiscountRate_1cf64481-3c4e-495e-99d7-df380c56c61b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccrualForEnvironmentalLossContingenciesDiscountRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_87183105-c160-4b0a-bc51-4405bee29a12" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingenciesDiscountRate_1cf64481-3c4e-495e-99d7-df380c56c61b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ScrapYardAndSolidWasteTransferStation_e36049dc-de5e-4325-9d94-831ec6d1926a" xlink:href="cwst-20200630.xsd#cwst_ScrapYardAndSolidWasteTransferStation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_87183105-c160-4b0a-bc51-4405bee29a12" xlink:to="loc_cwst_ScrapYardAndSolidWasteTransferStation_e36049dc-de5e-4325-9d94-831ec6d1926a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_UndiscountedCostsPreferredRemedies_ca6a331c-312b-45cf-8455-34f8a66fefdf" xlink:href="cwst-20200630.xsd#cwst_UndiscountedCostsPreferredRemedies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_87183105-c160-4b0a-bc51-4405bee29a12" xlink:to="loc_cwst_UndiscountedCostsPreferredRemedies_ca6a331c-312b-45cf-8455-34f8a66fefdf" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_PercentageParticipationInOnsiteRemediationActivities_a79cd4f7-8e3f-4fed-839d-d93509c4fd34" xlink:href="cwst-20200630.xsd#cwst_PercentageParticipationInOnsiteRemediationActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_87183105-c160-4b0a-bc51-4405bee29a12" xlink:to="loc_cwst_PercentageParticipationInOnsiteRemediationActivities_a79cd4f7-8e3f-4fed-839d-d93509c4fd34" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingencies_6126cdf2-9aac-44d6-b3d3-242430c74175" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccrualForEnvironmentalLossContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_87183105-c160-4b0a-bc51-4405bee29a12" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingencies_6126cdf2-9aac-44d6-b3d3-242430c74175" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyNumberOfPlaintiffs_c4d4846d-dd48-47dd-b8e3-4cbe03dab1ef" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LossContingencyNumberOfPlaintiffs"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_87183105-c160-4b0a-bc51-4405bee29a12" xlink:to="loc_us-gaap_LossContingencyNumberOfPlaintiffs_c4d4846d-dd48-47dd-b8e3-4cbe03dab1ef" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_ec0e10ce-5966-43bc-aa05-870aa54df840" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_87183105-c160-4b0a-bc51-4405bee29a12" xlink:to="loc_us-gaap_LossContingenciesTable_ec0e10ce-5966-43bc-aa05-870aa54df840" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseAxis_956ea739-2495-439e-85f4-68e89f435333" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_LitigationCaseAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_ec0e10ce-5966-43bc-aa05-870aa54df840" xlink:to="loc_srt_LitigationCaseAxis_956ea739-2495-439e-85f4-68e89f435333" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_956ea739-2495-439e-85f4-68e89f435333_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_LitigationCaseAxis_956ea739-2495-439e-85f4-68e89f435333" xlink:to="loc_srt_LitigationCaseTypeDomain_956ea739-2495-439e-85f4-68e89f435333_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_ada7521c-2cb6-4a17-9d98-38c924b50992" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_LitigationCaseAxis_956ea739-2495-439e-85f4-68e89f435333" xlink:to="loc_srt_LitigationCaseTypeDomain_ada7521c-2cb6-4a17-9d98-38c924b50992" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NoticeofIntenttoSueundertheResourceConservationandRecoveryActMember_464bfd25-b5cf-4523-9c59-dece1c3a8bb9" xlink:href="cwst-20200630.xsd#cwst_NoticeofIntenttoSueundertheResourceConservationandRecoveryActMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_ada7521c-2cb6-4a17-9d98-38c924b50992" xlink:to="loc_cwst_NoticeofIntenttoSueundertheResourceConservationandRecoveryActMember_464bfd25-b5cf-4523-9c59-dece1c3a8bb9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_OntarioCountyNewYorkClassActionLitigationMember_68ea7af9-ebeb-44d4-89dd-5062ede0827a" xlink:href="cwst-20200630.xsd#cwst_OntarioCountyNewYorkClassActionLitigationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_ada7521c-2cb6-4a17-9d98-38c924b50992" xlink:to="loc_cwst_OntarioCountyNewYorkClassActionLitigationMember_68ea7af9-ebeb-44d4-89dd-5062ede0827a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NoticeOfIntentToSueForViolationsOfTheCWAMember_be908536-e091-47d8-964f-caaac05155b8" xlink:href="cwst-20200630.xsd#cwst_NoticeOfIntentToSueForViolationsOfTheCWAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_ada7521c-2cb6-4a17-9d98-38c924b50992" xlink:to="loc_cwst_NoticeOfIntentToSueForViolationsOfTheCWAMember_be908536-e091-47d8-964f-caaac05155b8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_e37e5440-3f3a-442c-8be2-8505fb5fcd5e" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_ec0e10ce-5966-43bc-aa05-870aa54df840" xlink:to="loc_dei_LegalEntityAxis_e37e5440-3f3a-442c-8be2-8505fb5fcd5e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_e37e5440-3f3a-442c-8be2-8505fb5fcd5e_default" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_e37e5440-3f3a-442c-8be2-8505fb5fcd5e" xlink:to="loc_dei_EntityDomain_e37e5440-3f3a-442c-8be2-8505fb5fcd5e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_cb5fc67d-3b86-4228-8135-4c731916f34d" xlink:href="https://xbrl.sec.gov/dei/2019/dei-2019-01-31.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_e37e5440-3f3a-442c-8be2-8505fb5fcd5e" xlink:to="loc_dei_EntityDomain_cb5fc67d-3b86-4228-8135-4c731916f34d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_MassachusettsDepartmentofEnvironmentalProtectionMADEPandCasellaWasteSystemsMember_49f630de-0023-409b-9edc-ebb01544ab2a" xlink:href="cwst-20200630.xsd#cwst_MassachusettsDepartmentofEnvironmentalProtectionMADEPandCasellaWasteSystemsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_cb5fc67d-3b86-4228-8135-4c731916f34d" xlink:to="loc_cwst_MassachusettsDepartmentofEnvironmentalProtectionMADEPandCasellaWasteSystemsMember_49f630de-0023-409b-9edc-ebb01544ab2a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_8c3fdb84-d817-4934-8f2a-26db84acdd30" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_ec0e10ce-5966-43bc-aa05-870aa54df840" xlink:to="loc_us-gaap_DebtInstrumentAxis_8c3fdb84-d817-4934-8f2a-26db84acdd30" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_8c3fdb84-d817-4934-8f2a-26db84acdd30_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_8c3fdb84-d817-4934-8f2a-26db84acdd30" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_8c3fdb84-d817-4934-8f2a-26db84acdd30_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_4db781ff-9345-4121-973f-c3046d039e71" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_8c3fdb84-d817-4934-8f2a-26db84acdd30" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_4db781ff-9345-4121-973f-c3046d039e71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_A21ESettlementandWaterSystemConstructionFundingAgreementMember_9bee8dab-8896-4f3f-a143-6be63c751e93" xlink:href="cwst-20200630.xsd#cwst_A21ESettlementandWaterSystemConstructionFundingAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_4db781ff-9345-4121-973f-c3046d039e71" xlink:to="loc_cwst_A21ESettlementandWaterSystemConstructionFundingAgreementMember_9bee8dab-8896-4f3f-a143-6be63c751e93" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_84efd5a0-55ad-43c4-a0ce-b8f6b0e86039" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_ec0e10ce-5966-43bc-aa05-870aa54df840" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_84efd5a0-55ad-43c4-a0ce-b8f6b0e86039" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_84efd5a0-55ad-43c4-a0ce-b8f6b0e86039_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_84efd5a0-55ad-43c4-a0ce-b8f6b0e86039" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_84efd5a0-55ad-43c4-a0ce-b8f6b0e86039_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_3ed21bfb-36c5-4468-876b-4a2a71629a6f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_84efd5a0-55ad-43c4-a0ce-b8f6b0e86039" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_3ed21bfb-36c5-4468-876b-4a2a71629a6f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BondsMember_6d26eac7-5ea3-4371-a9a8-84b922c9ba3d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_BondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_3ed21bfb-36c5-4468-876b-4a2a71629a6f" xlink:to="loc_us-gaap_BondsMember_6d26eac7-5ea3-4371-a9a8-84b922c9ba3d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_c146a9ea-657b-4077-88a0-50a74cdb555f" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_ec0e10ce-5966-43bc-aa05-870aa54df840" xlink:to="loc_srt_RangeAxis_c146a9ea-657b-4077-88a0-50a74cdb555f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_c146a9ea-657b-4077-88a0-50a74cdb555f_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_c146a9ea-657b-4077-88a0-50a74cdb555f" xlink:to="loc_srt_RangeMember_c146a9ea-657b-4077-88a0-50a74cdb555f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_75cbe278-a34b-4bf8-83d7-ebf3ad0fb6da" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_c146a9ea-657b-4077-88a0-50a74cdb555f" xlink:to="loc_srt_RangeMember_75cbe278-a34b-4bf8-83d7-ebf3ad0fb6da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_159edda7-e798-47d9-baf3-c3dbc4274c0f" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_75cbe278-a34b-4bf8-83d7-ebf3ad0fb6da" xlink:to="loc_srt_MaximumMember_159edda7-e798-47d9-baf3-c3dbc4274c0f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationSiteAxis_021000b2-32ed-4002-9046-b027fceab6a8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EnvironmentalRemediationSiteAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_ec0e10ce-5966-43bc-aa05-870aa54df840" xlink:to="loc_us-gaap_EnvironmentalRemediationSiteAxis_021000b2-32ed-4002-9046-b027fceab6a8" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationSiteDomain_021000b2-32ed-4002-9046-b027fceab6a8_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EnvironmentalRemediationSiteDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteAxis_021000b2-32ed-4002-9046-b027fceab6a8" xlink:to="loc_us-gaap_EnvironmentalRemediationSiteDomain_021000b2-32ed-4002-9046-b027fceab6a8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationSiteDomain_3e387ce8-cdaf-424f-b060-0522bc336bcc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EnvironmentalRemediationSiteDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteAxis_021000b2-32ed-4002-9046-b027fceab6a8" xlink:to="loc_us-gaap_EnvironmentalRemediationSiteDomain_3e387ce8-cdaf-424f-b060-0522bc336bcc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SouthbridgeLandfillMember_5501bce1-f48d-4552-83ee-0504580f58c7" xlink:href="cwst-20200630.xsd#cwst_SouthbridgeLandfillMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteDomain_3e387ce8-cdaf-424f-b060-0522bc336bcc" xlink:to="loc_cwst_SouthbridgeLandfillMember_5501bce1-f48d-4552-83ee-0504580f58c7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_PotsdamEnvironmentalLiabilityMember_79f847a5-7abc-4610-adf1-028fb6a504a2" xlink:href="cwst-20200630.xsd#cwst_PotsdamEnvironmentalLiabilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteDomain_3e387ce8-cdaf-424f-b060-0522bc336bcc" xlink:to="loc_cwst_PotsdamEnvironmentalLiabilityMember_79f847a5-7abc-4610-adf1-028fb6a504a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NCESLandfilMember_30f1b431-7067-4804-b46a-04fe5e6d11eb" xlink:href="cwst-20200630.xsd#cwst_NCESLandfilMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteDomain_3e387ce8-cdaf-424f-b060-0522bc336bcc" xlink:to="loc_cwst_NCESLandfilMember_30f1b431-7067-4804-b46a-04fe5e6d11eb" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/COMMITMENTSANDCONTINGENCIESEnvironmentalRemediationLiabilityDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#COMMITMENTSANDCONTINGENCIESEnvironmentalRemediationLiabilityDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/COMMITMENTSANDCONTINGENCIESEnvironmentalRemediationLiabilityDetails" xlink:type="extended" id="iba11d517a61145cfbc9767a34c3f4da4_COMMITMENTSANDCONTINGENCIESEnvironmentalRemediationLiabilityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SiteContingencyLineItems_59e7693a-db6d-41db-928f-5026dd374f6d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SiteContingencyLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingenciesRollForward_e98af604-a1f7-44df-9389-ba59b0de2cda" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccrualForEnvironmentalLossContingenciesRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SiteContingencyLineItems_59e7693a-db6d-41db-928f-5026dd374f6d" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingenciesRollForward_e98af604-a1f7-44df-9389-ba59b0de2cda" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingencies_daccafe1-cf3c-4cef-8aca-15f9a0730b70" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccrualForEnvironmentalLossContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccrualForEnvironmentalLossContingenciesRollForward_e98af604-a1f7-44df-9389-ba59b0de2cda" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingencies_daccafe1-cf3c-4cef-8aca-15f9a0730b70" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingenciesChargesToExpenseForNewLosses_382d49c8-8b31-4ed0-90e8-75f669febd5c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccrualForEnvironmentalLossContingenciesChargesToExpenseForNewLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccrualForEnvironmentalLossContingenciesRollForward_e98af604-a1f7-44df-9389-ba59b0de2cda" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingenciesChargesToExpenseForNewLosses_382d49c8-8b31-4ed0-90e8-75f669febd5c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_AccrualForEnvironmentalLossContingenciesObligationsIncurred_fe198e2e-15dc-4267-85f5-fdf3658593fa" xlink:href="cwst-20200630.xsd#cwst_AccrualForEnvironmentalLossContingenciesObligationsIncurred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccrualForEnvironmentalLossContingenciesRollForward_e98af604-a1f7-44df-9389-ba59b0de2cda" xlink:to="loc_cwst_AccrualForEnvironmentalLossContingenciesObligationsIncurred_fe198e2e-15dc-4267-85f5-fdf3658593fa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingenciesIncreaseDecreaseForRevisionInEstimates_93ec4c1f-9b51-49e0-9bc1-0122e85fb83e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccrualForEnvironmentalLossContingenciesIncreaseDecreaseForRevisionInEstimates"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccrualForEnvironmentalLossContingenciesRollForward_e98af604-a1f7-44df-9389-ba59b0de2cda" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingenciesIncreaseDecreaseForRevisionInEstimates_93ec4c1f-9b51-49e0-9bc1-0122e85fb83e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingenciesPayments1_f03f3015-c9f6-435f-ada6-8fdcb02efebf" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccrualForEnvironmentalLossContingenciesPayments1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccrualForEnvironmentalLossContingenciesRollForward_e98af604-a1f7-44df-9389-ba59b0de2cda" xlink:to="loc_us-gaap_AccrualForEnvironmentalLossContingenciesPayments1_f03f3015-c9f6-435f-ada6-8fdcb02efebf" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccrualForEnvironmentalLossContingencies_f0a394f4-de93-4647-acde-3ae25d9e44ff" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccrualForEnvironmentalLossContingencies"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SiteContingencyTable_e2c40393-b3e7-4920-afb7-ba335af327fe" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SiteContingencyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SiteContingencyLineItems_59e7693a-db6d-41db-928f-5026dd374f6d" xlink:to="loc_us-gaap_SiteContingencyTable_e2c40393-b3e7-4920-afb7-ba335af327fe" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationSiteAxis_3d32b971-0418-4b33-a9e1-a1ff937f1c04" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EnvironmentalRemediationSiteAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SiteContingencyTable_e2c40393-b3e7-4920-afb7-ba335af327fe" xlink:to="loc_us-gaap_EnvironmentalRemediationSiteAxis_3d32b971-0418-4b33-a9e1-a1ff937f1c04" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationSiteDomain_3d32b971-0418-4b33-a9e1-a1ff937f1c04_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EnvironmentalRemediationSiteDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteAxis_3d32b971-0418-4b33-a9e1-a1ff937f1c04" xlink:to="loc_us-gaap_EnvironmentalRemediationSiteDomain_3d32b971-0418-4b33-a9e1-a1ff937f1c04_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationSiteDomain_1da5a7e5-d33b-42ac-87a5-6db5fc1a01db" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EnvironmentalRemediationSiteDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteAxis_3d32b971-0418-4b33-a9e1-a1ff937f1c04" xlink:to="loc_us-gaap_EnvironmentalRemediationSiteDomain_1da5a7e5-d33b-42ac-87a5-6db5fc1a01db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SouthbridgeLandfillMember_f20fd305-27e9-4fa2-b775-1473f0f1ae11" xlink:href="cwst-20200630.xsd#cwst_SouthbridgeLandfillMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteDomain_1da5a7e5-d33b-42ac-87a5-6db5fc1a01db" xlink:to="loc_cwst_SouthbridgeLandfillMember_f20fd305-27e9-4fa2-b775-1473f0f1ae11" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="cwst-20200630.xsd#STOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITY" xlink:type="extended" id="i8213628226b74efc9eba06fb5dd3d114_STOCKHOLDERSEQUITY"/>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYTables" xlink:type="simple" xlink:href="cwst-20200630.xsd#STOCKHOLDERSEQUITYTables"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYTables" xlink:type="extended" id="i6baede9ad0d94929ad8087c2d5085724_STOCKHOLDERSEQUITYTables"/>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYNarrativeDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#STOCKHOLDERSEQUITYNarrativeDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYNarrativeDetail" xlink:type="extended" id="if4c1552bf1b24ed282015c81bf170de6_STOCKHOLDERSEQUITYNarrativeDetail">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_4489887b-3924-45df-b1dd-b21180a72ee6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LimitedPartnersCapitalAccountLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_7e179c06-6531-4d66-ad4f-3c57adc04d25" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_4489887b-3924-45df-b1dd-b21180a72ee6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_7e179c06-6531-4d66-ad4f-3c57adc04d25" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_079274fc-3ac3-43fb-b8c9-e060db13e9da" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_4489887b-3924-45df-b1dd-b21180a72ee6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized_079274fc-3ac3-43fb-b8c9-e060db13e9da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_8a572733-1d13-400b-83da-0c02c6db153d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_4489887b-3924-45df-b1dd-b21180a72ee6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_8a572733-1d13-400b-83da-0c02c6db153d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_19d91934-85cb-49b2-bd60-7fac8d8a4342" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_4489887b-3924-45df-b1dd-b21180a72ee6" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_19d91934-85cb-49b2-bd60-7fac8d8a4342" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_6709ecad-6c94-42dc-8fdf-ff27b6e19ed6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_4489887b-3924-45df-b1dd-b21180a72ee6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_6709ecad-6c94-42dc-8fdf-ff27b6e19ed6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_566ac978-b43e-4024-8e72-43996a239fdd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_4489887b-3924-45df-b1dd-b21180a72ee6" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_566ac978-b43e-4024-8e72-43996a239fdd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_91085ed2-a010-42b6-9a53-c539d443a5f5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_4489887b-3924-45df-b1dd-b21180a72ee6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_91085ed2-a010-42b6-9a53-c539d443a5f5" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_4ebbb06d-2b45-4572-b963-d1ad7eda8cf7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_4489887b-3924-45df-b1dd-b21180a72ee6" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_4ebbb06d-2b45-4572-b963-d1ad7eda8cf7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_f4ec9187-4d54-4e95-b5a0-246a786ce884" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_4489887b-3924-45df-b1dd-b21180a72ee6" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_f4ec9187-4d54-4e95-b5a0-246a786ce884" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_2b939a0f-ac59-4430-b8bb-ddb37405d0bb" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_4489887b-3924-45df-b1dd-b21180a72ee6" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_2b939a0f-ac59-4430-b8bb-ddb37405d0bb" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_433c3edc-c2e9-4278-a2ae-0107a032d55a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LimitedPartnersCapitalAccountLineItems_4489887b-3924-45df-b1dd-b21180a72ee6" xlink:to="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_433c3edc-c2e9-4278-a2ae-0107a032d55a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_0971ec73-23ce-4ea8-9d23-c08f1966075b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_433c3edc-c2e9-4278-a2ae-0107a032d55a" xlink:to="loc_us-gaap_StatementClassOfStockAxis_0971ec73-23ce-4ea8-9d23-c08f1966075b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_0971ec73-23ce-4ea8-9d23-c08f1966075b_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_0971ec73-23ce-4ea8-9d23-c08f1966075b" xlink:to="loc_us-gaap_ClassOfStockDomain_0971ec73-23ce-4ea8-9d23-c08f1966075b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_fe5989c1-dbd4-4621-a413-0c598cf9fa7d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_0971ec73-23ce-4ea8-9d23-c08f1966075b" xlink:to="loc_us-gaap_ClassOfStockDomain_fe5989c1-dbd4-4621-a413-0c598cf9fa7d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_75af1e1b-35f3-44e8-934c-a43e75af51c7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_fe5989c1-dbd4-4621-a413-0c598cf9fa7d" xlink:to="loc_us-gaap_CommonClassAMember_75af1e1b-35f3-44e8-934c-a43e75af51c7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_f34db05f-a75b-4c53-9527-786796b9a9bd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_433c3edc-c2e9-4278-a2ae-0107a032d55a" xlink:to="loc_us-gaap_PlanNameAxis_f34db05f-a75b-4c53-9527-786796b9a9bd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_f34db05f-a75b-4c53-9527-786796b9a9bd_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_f34db05f-a75b-4c53-9527-786796b9a9bd" xlink:to="loc_us-gaap_PlanNameDomain_f34db05f-a75b-4c53-9527-786796b9a9bd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_30a67ef5-5901-4638-8c0c-35a9eb114153" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_f34db05f-a75b-4c53-9527-786796b9a9bd" xlink:to="loc_us-gaap_PlanNameDomain_30a67ef5-5901-4638-8c0c-35a9eb114153" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_A2016IncentivePlanMember_735fc4e5-e0c6-474b-8aa3-7b8c1643869a" xlink:href="cwst-20200630.xsd#cwst_A2016IncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_30a67ef5-5901-4638-8c0c-35a9eb114153" xlink:to="loc_cwst_A2016IncentivePlanMember_735fc4e5-e0c6-474b-8aa3-7b8c1643869a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TwoThousandSixStockIncentivePlanMember_f8a2e891-54a0-467a-b768-3d382e8f4cb1" xlink:href="cwst-20200630.xsd#cwst_TwoThousandSixStockIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_30a67ef5-5901-4638-8c0c-35a9eb114153" xlink:to="loc_cwst_TwoThousandSixStockIncentivePlanMember_f8a2e891-54a0-467a-b768-3d382e8f4cb1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember_fe9d3c1d-d8f5-4b79-8e86-15cd1eca5d24" xlink:href="cwst-20200630.xsd#cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_30a67ef5-5901-4638-8c0c-35a9eb114153" xlink:to="loc_cwst_AmendedAndRestatedNineteenNinetySevenEmployeeStockPurchasePlanMember_fe9d3c1d-d8f5-4b79-8e86-15cd1eca5d24" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_31105c91-4c3e-42bd-90ba-77fb22268f6c" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_433c3edc-c2e9-4278-a2ae-0107a032d55a" xlink:to="loc_srt_RangeAxis_31105c91-4c3e-42bd-90ba-77fb22268f6c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_31105c91-4c3e-42bd-90ba-77fb22268f6c_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_31105c91-4c3e-42bd-90ba-77fb22268f6c" xlink:to="loc_srt_RangeMember_31105c91-4c3e-42bd-90ba-77fb22268f6c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_2d2723b4-b40a-4a24-8fb2-e695c6b88ba9" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_31105c91-4c3e-42bd-90ba-77fb22268f6c" xlink:to="loc_srt_RangeMember_2d2723b4-b40a-4a24-8fb2-e695c6b88ba9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_61f08851-2f85-4523-abb8-97e32a13927c" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_2d2723b4-b40a-4a24-8fb2-e695c6b88ba9" xlink:to="loc_srt_MinimumMember_61f08851-2f85-4523-abb8-97e32a13927c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_c2cd5554-e268-46b7-9a57-d7406c317622" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_2d2723b4-b40a-4a24-8fb2-e695c6b88ba9" xlink:to="loc_srt_MaximumMember_c2cd5554-e268-46b7-9a57-d7406c317622" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_WeightedAverageMember_0ab874e6-5173-42d5-af15-c71e7195f6bb" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_WeightedAverageMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_2d2723b4-b40a-4a24-8fb2-e695c6b88ba9" xlink:to="loc_srt_WeightedAverageMember_0ab874e6-5173-42d5-af15-c71e7195f6bb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_558bee84-c9a5-4e21-8a55-313ff9af1e45" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_433c3edc-c2e9-4278-a2ae-0107a032d55a" xlink:to="loc_us-gaap_AwardTypeAxis_558bee84-c9a5-4e21-8a55-313ff9af1e45" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_558bee84-c9a5-4e21-8a55-313ff9af1e45_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_558bee84-c9a5-4e21-8a55-313ff9af1e45" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_558bee84-c9a5-4e21-8a55-313ff9af1e45_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4d3d2c47-774c-4399-80e7-eb7e906c65f3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_558bee84-c9a5-4e21-8a55-313ff9af1e45" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4d3d2c47-774c-4399-80e7-eb7e906c65f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_657172ee-62da-4e18-9f5b-0953794f55e4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4d3d2c47-774c-4399-80e7-eb7e906c65f3" xlink:to="loc_us-gaap_EmployeeStockOptionMember_657172ee-62da-4e18-9f5b-0953794f55e4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_2bbf83de-05ed-44c1-a7b6-6209daaa3530" xlink:href="cwst-20200630.xsd#cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4d3d2c47-774c-4399-80e7-eb7e906c65f3" xlink:to="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_2bbf83de-05ed-44c1-a7b6-6209daaa3530" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RestrictedStockAwardsMember_93b3dacf-2dd9-4067-8f20-7d4f602b634d" xlink:href="cwst-20200630.xsd#cwst_RestrictedStockAwardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_2bbf83de-05ed-44c1-a7b6-6209daaa3530" xlink:to="loc_cwst_RestrictedStockAwardsMember_93b3dacf-2dd9-4067-8f20-7d4f602b634d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_4b26f303-39e0-45f4-a732-06d650c50af4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_2bbf83de-05ed-44c1-a7b6-6209daaa3530" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_4b26f303-39e0-45f4-a732-06d650c50af4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_PerformanceStockUnitMember_de0d2883-3013-479a-9d85-c57f341ea95a" xlink:href="cwst-20200630.xsd#cwst_PerformanceStockUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_2bbf83de-05ed-44c1-a7b6-6209daaa3530" xlink:to="loc_cwst_PerformanceStockUnitMember_de0d2883-3013-479a-9d85-c57f341ea95a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_80eb6f4e-955c-43ad-88f4-96279b5c8379" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfLimitedPartnersCapitalAccountByClassTable_433c3edc-c2e9-4278-a2ae-0107a032d55a" xlink:to="loc_srt_TitleOfIndividualAxis_80eb6f4e-955c-43ad-88f4-96279b5c8379" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_80eb6f4e-955c-43ad-88f4-96279b5c8379_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_80eb6f4e-955c-43ad-88f4-96279b5c8379" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_80eb6f4e-955c-43ad-88f4-96279b5c8379_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_bf29d84f-9aa4-421b-a37f-d2a0f1579204" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_80eb6f4e-955c-43ad-88f4-96279b5c8379" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_bf29d84f-9aa4-421b-a37f-d2a0f1579204" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_NonEmployeeDirectorMember_a8287c94-dcdc-4eda-a271-bc274b396324" xlink:href="cwst-20200630.xsd#cwst_NonEmployeeDirectorMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_bf29d84f-9aa4-421b-a37f-d2a0f1579204" xlink:to="loc_cwst_NonEmployeeDirectorMember_a8287c94-dcdc-4eda-a271-bc274b396324" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofStockOptionActivityDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#STOCKHOLDERSEQUITYSummaryofStockOptionActivityDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofStockOptionActivityDetail" xlink:type="extended" id="id0603ed491e94114bc62ef549d84804c_STOCKHOLDERSEQUITYSummaryofStockOptionActivityDetail"/>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofRestrictedStockRestrictedStockUnitandPerformancebasedStockUnitActivityDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#STOCKHOLDERSEQUITYSummaryofRestrictedStockRestrictedStockUnitandPerformancebasedStockUnitActivityDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofRestrictedStockRestrictedStockUnitandPerformancebasedStockUnitActivityDetail" xlink:type="extended" id="ib26d96f849884086a9022415f0387e1c_STOCKHOLDERSEQUITYSummaryofRestrictedStockRestrictedStockUnitandPerformancebasedStockUnitActivityDetail">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6c8bbaf0-ae69-470d-ae80-63084cf5155f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_db0955a0-ba7e-4499-9e1d-a0c91d4cb924" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6c8bbaf0-ae69-470d-ae80-63084cf5155f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_db0955a0-ba7e-4499-9e1d-a0c91d4cb924" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_a68019ae-ef5a-4f13-8fd6-6efe7324f569" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_db0955a0-ba7e-4499-9e1d-a0c91d4cb924" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_a68019ae-ef5a-4f13-8fd6-6efe7324f569" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_e9512ffd-c25f-4704-b7dc-a105ab17b016" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_db0955a0-ba7e-4499-9e1d-a0c91d4cb924" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_e9512ffd-c25f-4704-b7dc-a105ab17b016" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_339beffc-c34d-4aae-90b6-4646c17eb834" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_db0955a0-ba7e-4499-9e1d-a0c91d4cb924" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_339beffc-c34d-4aae-90b6-4646c17eb834" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_93e4a12b-4a2c-421e-b945-b0bcfd394b65" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_db0955a0-ba7e-4499-9e1d-a0c91d4cb924" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_93e4a12b-4a2c-421e-b945-b0bcfd394b65" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_20ba4aa9-e28c-4316-a70a-5b730ac3127c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_7eb4066a-8c5c-49e9-8849-b56caed567fd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6c8bbaf0-ae69-470d-ae80-63084cf5155f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_7eb4066a-8c5c-49e9-8849-b56caed567fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_bcb60f75-06ad-4e89-a2eb-26a9a165b163" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_7eb4066a-8c5c-49e9-8849-b56caed567fd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_bcb60f75-06ad-4e89-a2eb-26a9a165b163" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_d026c461-6b6f-4a39-832a-afa3e0ccc7d3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_7eb4066a-8c5c-49e9-8849-b56caed567fd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_d026c461-6b6f-4a39-832a-afa3e0ccc7d3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_0b6f67dd-0436-4992-a6f7-27628d3ae87c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_7eb4066a-8c5c-49e9-8849-b56caed567fd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_0b6f67dd-0436-4992-a6f7-27628d3ae87c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_ff555670-f4b8-45a8-8ecf-414ceeffa50e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_7eb4066a-8c5c-49e9-8849-b56caed567fd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_ff555670-f4b8-45a8-8ecf-414ceeffa50e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_d384d21e-6ed4-4ecf-af38-8145080fa35e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_fe901a07-0156-4092-ac10-1d08da3a336c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6c8bbaf0-ae69-470d-ae80-63084cf5155f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_fe901a07-0156-4092-ac10-1d08da3a336c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_346c4be5-b0fe-4a58-a974-8005f5dfbea4" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_fe901a07-0156-4092-ac10-1d08da3a336c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms_346c4be5-b0fe-4a58-a974-8005f5dfbea4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_73ab803f-35a1-44d3-9cef-4fba7c37503c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract_fe901a07-0156-4092-ac10-1d08da3a336c" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueOutstanding_73ab803f-35a1-44d3-9cef-4fba7c37503c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_0d4eb008-e315-4891-a81c-9389df4c4b3e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6c8bbaf0-ae69-470d-ae80-63084cf5155f" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_0d4eb008-e315-4891-a81c-9389df4c4b3e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_AdditionalSharesIssuedMaximumAttainmentLevel_e4ef96b5-e3ea-4c07-bdfa-4f4aa08216cd" xlink:href="cwst-20200630.xsd#cwst_AdditionalSharesIssuedMaximumAttainmentLevel"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6c8bbaf0-ae69-470d-ae80-63084cf5155f" xlink:to="loc_cwst_AdditionalSharesIssuedMaximumAttainmentLevel_e4ef96b5-e3ea-4c07-bdfa-4f4aa08216cd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_6c37e4f7-fd82-43ed-a55a-cb9de3b9525c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6c8bbaf0-ae69-470d-ae80-63084cf5155f" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_6c37e4f7-fd82-43ed-a55a-cb9de3b9525c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_e2389850-e534-437e-af48-7fccc7d43e85" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_6c37e4f7-fd82-43ed-a55a-cb9de3b9525c" xlink:to="loc_us-gaap_VestingAxis_e2389850-e534-437e-af48-7fccc7d43e85" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_e2389850-e534-437e-af48-7fccc7d43e85_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_e2389850-e534-437e-af48-7fccc7d43e85" xlink:to="loc_us-gaap_VestingDomain_e2389850-e534-437e-af48-7fccc7d43e85_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_341dbe47-cfda-46d9-b471-c94b6cb4e26b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_e2389850-e534-437e-af48-7fccc7d43e85" xlink:to="loc_us-gaap_VestingDomain_341dbe47-cfda-46d9-b471-c94b6cb4e26b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_6ae2de6d-f800-48b7-b9e2-e0258949e852" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_341dbe47-cfda-46d9-b471-c94b6cb4e26b" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_6ae2de6d-f800-48b7-b9e2-e0258949e852" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_176dac56-3ac8-45fe-be89-6fb878da6eb9" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_6c37e4f7-fd82-43ed-a55a-cb9de3b9525c" xlink:to="loc_us-gaap_AwardTypeAxis_176dac56-3ac8-45fe-be89-6fb878da6eb9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_176dac56-3ac8-45fe-be89-6fb878da6eb9_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_176dac56-3ac8-45fe-be89-6fb878da6eb9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_176dac56-3ac8-45fe-be89-6fb878da6eb9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ef9e6af3-9bc2-4b0e-9309-502f5925e200" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_176dac56-3ac8-45fe-be89-6fb878da6eb9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ef9e6af3-9bc2-4b0e-9309-502f5925e200" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_7337a64a-e3b7-4960-9226-83fc5d9492d8" xlink:href="cwst-20200630.xsd#cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ef9e6af3-9bc2-4b0e-9309-502f5925e200" xlink:to="loc_cwst_RestrictedStockRestrictedStockUnitsAndPerformanceStockUnitsMember_7337a64a-e3b7-4960-9226-83fc5d9492d8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_PerformanceStockUnitMember_e0573102-e42d-4521-ac7c-be2fc4471b99" xlink:href="cwst-20200630.xsd#cwst_PerformanceStockUnitMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ef9e6af3-9bc2-4b0e-9309-502f5925e200" xlink:to="loc_cwst_PerformanceStockUnitMember_e0573102-e42d-4521-ac7c-be2fc4471b99" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_c5f7dff9-ffcc-4d63-b6c3-b6439c4bcdc6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_6c37e4f7-fd82-43ed-a55a-cb9de3b9525c" xlink:to="loc_us-gaap_StatementClassOfStockAxis_c5f7dff9-ffcc-4d63-b6c3-b6439c4bcdc6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_c5f7dff9-ffcc-4d63-b6c3-b6439c4bcdc6_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_c5f7dff9-ffcc-4d63-b6c3-b6439c4bcdc6" xlink:to="loc_us-gaap_ClassOfStockDomain_c5f7dff9-ffcc-4d63-b6c3-b6439c4bcdc6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_6dc868ce-df73-4c0f-a4d6-0631e9f7147f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_c5f7dff9-ffcc-4d63-b6c3-b6439c4bcdc6" xlink:to="loc_us-gaap_ClassOfStockDomain_6dc868ce-df73-4c0f-a4d6-0631e9f7147f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_211f8265-2e69-42af-bf96-f32acca4a73c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_6dc868ce-df73-4c0f-a4d6-0631e9f7147f" xlink:to="loc_us-gaap_CommonClassAMember_211f8265-2e69-42af-bf96-f32acca4a73c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofChangesinBalancesofEachComponentofAccumulatedOtherComprehensiveIncomeDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#STOCKHOLDERSEQUITYSummaryofChangesinBalancesofEachComponentofAccumulatedOtherComprehensiveIncomeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofChangesinBalancesofEachComponentofAccumulatedOtherComprehensiveIncomeDetails" xlink:type="extended" id="ib59a15d7d80741cfa0fa5df4337c2928_STOCKHOLDERSEQUITYSummaryofChangesinBalancesofEachComponentofAccumulatedOtherComprehensiveIncomeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_b4a09f94-5c0f-4b6a-a78e-2b9dba3e46d6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_6d328158-1794-441f-b030-767a3d4868de" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_b4a09f94-5c0f-4b6a-a78e-2b9dba3e46d6" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_6d328158-1794-441f-b030-767a3d4868de" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_686b7f79-8df6-4f14-9a1f-90a90da322ea" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_6d328158-1794-441f-b030-767a3d4868de" xlink:to="loc_us-gaap_StockholdersEquity_686b7f79-8df6-4f14-9a1f-90a90da322ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_fce5fe74-1334-48b0-9e6b-0786e7141623" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_6d328158-1794-441f-b030-767a3d4868de" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax_fce5fe74-1334-48b0-9e6b-0786e7141623" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_1a4a7c27-52c9-4d3f-824d-2236327888d2" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_6d328158-1794-441f-b030-767a3d4868de" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax_1a4a7c27-52c9-4d3f-824d-2236327888d2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_9022f332-91ee-410d-8c08-6debf9dcc5ff" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_6d328158-1794-441f-b030-767a3d4868de" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_9022f332-91ee-410d-8c08-6debf9dcc5ff" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_7de546fa-5f9b-4db5-967a-029f297ff40a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTaxAbstract_6d328158-1794-441f-b030-767a3d4868de" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_7de546fa-5f9b-4db5-967a-029f297ff40a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_a790f5b4-dbf4-4efc-b58a-5101f72ed193" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_91ffade6-08d8-46d0-abe3-0fdc70c1d6cc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_b4a09f94-5c0f-4b6a-a78e-2b9dba3e46d6" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_91ffade6-08d8-46d0-abe3-0fdc70c1d6cc" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_aeb14420-9ad0-47c3-a0e6-a27498be08c6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_91ffade6-08d8-46d0-abe3-0fdc70c1d6cc" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_aeb14420-9ad0-47c3-a0e6-a27498be08c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_aeb14420-9ad0-47c3-a0e6-a27498be08c6_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_aeb14420-9ad0-47c3-a0e6-a27498be08c6" xlink:to="loc_us-gaap_EquityComponentDomain_aeb14420-9ad0-47c3-a0e6-a27498be08c6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_6042937c-29ec-4836-95cc-b231b74532c6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_aeb14420-9ad0-47c3-a0e6-a27498be08c6" xlink:to="loc_us-gaap_EquityComponentDomain_6042937c-29ec-4836-95cc-b231b74532c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember_aa858e11-c592-49e1-8689-872910b5bfa0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_6042937c-29ec-4836-95cc-b231b74532c6" xlink:to="loc_us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember_aa858e11-c592-49e1-8689-872910b5bfa0" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofReclassificationsOutofAccumulatedOtherComprehensiveIncomeDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#STOCKHOLDERSEQUITYSummaryofReclassificationsOutofAccumulatedOtherComprehensiveIncomeDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/STOCKHOLDERSEQUITYSummaryofReclassificationsOutofAccumulatedOtherComprehensiveIncomeDetails" xlink:type="extended" id="i402be4bdd12f46819f9b428c2cbe4310_STOCKHOLDERSEQUITYSummaryofReclassificationsOutofAccumulatedOtherComprehensiveIncomeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_7f992655-8e75-476a-b650-2fff7a124db7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_0ee4430a-ef2a-4b73-a676-01367aad3a31" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_7f992655-8e75-476a-b650-2fff7a124db7" xlink:to="loc_us-gaap_InterestExpense_0ee4430a-ef2a-4b73-a676-01367aad3a31" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_2baec0f8-134a-45d6-b12a-7776d61c8437" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_7f992655-8e75-476a-b650-2fff7a124db7" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_2baec0f8-134a-45d6-b12a-7776d61c8437" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_f8014777-4c10-4ba7-af07-73f7f6fb8609" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_7f992655-8e75-476a-b650-2fff7a124db7" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_f8014777-4c10-4ba7-af07-73f7f6fb8609" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_3fd12f7b-f390-45fe-bcf0-ef6c8907b443" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_7f992655-8e75-476a-b650-2fff7a124db7" xlink:to="loc_us-gaap_NetIncomeLoss_3fd12f7b-f390-45fe-bcf0-ef6c8907b443" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_ebf09ee4-069e-4378-8bb6-236ef06be095" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_7f992655-8e75-476a-b650-2fff7a124db7" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_ebf09ee4-069e-4378-8bb6-236ef06be095" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeInstrumentRiskAxis_3c09a4e6-933c-40f2-b1ac-df88e44fe30d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_ebf09ee4-069e-4378-8bb6-236ef06be095" xlink:to="loc_us-gaap_DerivativeInstrumentRiskAxis_3c09a4e6-933c-40f2-b1ac-df88e44fe30d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_3c09a4e6-933c-40f2-b1ac-df88e44fe30d_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_3c09a4e6-933c-40f2-b1ac-df88e44fe30d" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_3c09a4e6-933c-40f2-b1ac-df88e44fe30d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeContractTypeDomain_9b1e8202-5628-459e-b152-b71f96376476" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DerivativeContractTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DerivativeInstrumentRiskAxis_3c09a4e6-933c-40f2-b1ac-df88e44fe30d" xlink:to="loc_us-gaap_DerivativeContractTypeDomain_9b1e8202-5628-459e-b152-b71f96376476" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateSwapMember_385fbc76-61da-4411-96dd-732829e205d1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InterestRateSwapMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeContractTypeDomain_9b1e8202-5628-459e-b152-b71f96376476" xlink:to="loc_us-gaap_InterestRateSwapMember_385fbc76-61da-4411-96dd-732829e205d1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_5bc1ce3e-0465-4548-8e79-7fe641751537" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_ebf09ee4-069e-4378-8bb6-236ef06be095" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_5bc1ce3e-0465-4548-8e79-7fe641751537" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_5bc1ce3e-0465-4548-8e79-7fe641751537_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_5bc1ce3e-0465-4548-8e79-7fe641751537" xlink:to="loc_us-gaap_EquityComponentDomain_5bc1ce3e-0465-4548-8e79-7fe641751537_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_1fc5fe73-1399-4a0b-83cd-a882230a3d78" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_5bc1ce3e-0465-4548-8e79-7fe641751537" xlink:to="loc_us-gaap_EquityComponentDomain_1fc5fe73-1399-4a0b-83cd-a882230a3d78" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember_4d7124fd-bcba-4318-8169-57ba3a894ddb" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_1fc5fe73-1399-4a0b-83cd-a882230a3d78" xlink:to="loc_us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember_4d7124fd-bcba-4318-8169-57ba3a894ddb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_3d3d33e6-38cb-45ff-92ed-76192231da01" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_ebf09ee4-069e-4378-8bb6-236ef06be095" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_3d3d33e6-38cb-45ff-92ed-76192231da01" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_3d3d33e6-38cb-45ff-92ed-76192231da01_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_3d3d33e6-38cb-45ff-92ed-76192231da01" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_3d3d33e6-38cb-45ff-92ed-76192231da01_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_69a2b2c5-2410-4344-b5d4-e8d300ccc886" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_3d3d33e6-38cb-45ff-92ed-76192231da01" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_69a2b2c5-2410-4344-b5d4-e8d300ccc886" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_c64dd591-3894-40b9-9512-9d9acf76f34e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_69a2b2c5-2410-4344-b5d4-e8d300ccc886" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_c64dd591-3894-40b9-9512-9d9acf76f34e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/EARNINGSPERSHARE" xlink:type="simple" xlink:href="cwst-20200630.xsd#EARNINGSPERSHARE"/>
  <link:definitionLink xlink:role="http://casella.com/role/EARNINGSPERSHARE" xlink:type="extended" id="i2f08d6438d2b4a13913587501ae7919a_EARNINGSPERSHARE"/>
  <link:roleRef roleURI="http://casella.com/role/EARNINGSPERSHARETables" xlink:type="simple" xlink:href="cwst-20200630.xsd#EARNINGSPERSHARETables"/>
  <link:definitionLink xlink:role="http://casella.com/role/EARNINGSPERSHARETables" xlink:type="extended" id="ib3dcf72de4e44acd96e36fc3bc15cdd1_EARNINGSPERSHARETables"/>
  <link:roleRef roleURI="http://casella.com/role/EARNINGSPERSHAREDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#EARNINGSPERSHAREDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/EARNINGSPERSHAREDetails" xlink:type="extended" id="if81b34aef29341d7acc6463d93b15a13_EARNINGSPERSHAREDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_d85ce7f9-89a4-42b6-bd3a-e29390e93784" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_f6a2dc27-4d3c-4436-9e80-ccaa6be41df3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_d85ce7f9-89a4-42b6-bd3a-e29390e93784" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_f6a2dc27-4d3c-4436-9e80-ccaa6be41df3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperations_1a136732-58a1-4c8b-8074-1272a835dcd5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncomeLossFromContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDilutedAbstract_f6a2dc27-4d3c-4436-9e80-ccaa6be41df3" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperations_1a136732-58a1-4c8b-8074-1272a835dcd5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_0819d732-2a88-4ede-94ce-f527d2ee8675" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_d85ce7f9-89a4-42b6-bd3a-e29390e93784" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_0819d732-2a88-4ede-94ce-f527d2ee8675" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_cccbdd3f-676b-41f5-a816-fead4c9b7b47" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_0819d732-2a88-4ede-94ce-f527d2ee8675" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_cccbdd3f-676b-41f5-a816-fead4c9b7b47" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesContingentlyIssuable_3be5076a-7ea9-4c1b-883d-90d3ae558161" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesContingentlyIssuable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_0819d732-2a88-4ede-94ce-f527d2ee8675" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesContingentlyIssuable_3be5076a-7ea9-4c1b-883d-90d3ae558161" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesRestrictedStock_232524e0-b7ae-4a07-95be-4b7b0860f27b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesRestrictedStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_0819d732-2a88-4ede-94ce-f527d2ee8675" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesRestrictedStock_232524e0-b7ae-4a07-95be-4b7b0860f27b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesIssuedBasic_a93b9250-84ad-4a48-8787-ceb9ff724275" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesIssuedBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingDilutedDisclosureItemsAbstract_0819d732-2a88-4ede-94ce-f527d2ee8675" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesIssuedBasic_a93b9250-84ad-4a48-8787-ceb9ff724275" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_61013480-f633-49a0-b73f-35090efedadc" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_d85ce7f9-89a4-42b6-bd3a-e29390e93784" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_61013480-f633-49a0-b73f-35090efedadc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_93833e68-f55c-43ce-acea-e1e6b7211b2c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_d85ce7f9-89a4-42b6-bd3a-e29390e93784" xlink:to="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_93833e68-f55c-43ce-acea-e1e6b7211b2c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_87741a31-b5f9-4b0f-88d8-7083ee3b5962" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_93833e68-f55c-43ce-acea-e1e6b7211b2c" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_87741a31-b5f9-4b0f-88d8-7083ee3b5962" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_6e2cee62-44a3-48a3-a928-a3232b1c7350" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_d85ce7f9-89a4-42b6-bd3a-e29390e93784" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_6e2cee62-44a3-48a3-a928-a3232b1c7350" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_709e6dd4-7f46-403b-b091-0054d218b6be" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_d85ce7f9-89a4-42b6-bd3a-e29390e93784" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_709e6dd4-7f46-403b-b091-0054d218b6be" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_7da30e82-26c2-4ee9-88e1-695cbd2535a1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_d85ce7f9-89a4-42b6-bd3a-e29390e93784" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_7da30e82-26c2-4ee9-88e1-695cbd2535a1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_9ea2569e-93c4-42f2-acad-72c0775296c0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_7da30e82-26c2-4ee9-88e1-695cbd2535a1" xlink:to="loc_us-gaap_StatementClassOfStockAxis_9ea2569e-93c4-42f2-acad-72c0775296c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_9ea2569e-93c4-42f2-acad-72c0775296c0_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_9ea2569e-93c4-42f2-acad-72c0775296c0" xlink:to="loc_us-gaap_ClassOfStockDomain_9ea2569e-93c4-42f2-acad-72c0775296c0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_b5044cef-9f1f-4ea9-9eac-9e9a92715690" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_9ea2569e-93c4-42f2-acad-72c0775296c0" xlink:to="loc_us-gaap_ClassOfStockDomain_b5044cef-9f1f-4ea9-9eac-9e9a92715690" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_9c8ac3c8-e6a4-469d-b04d-4dc24829ac77" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_b5044cef-9f1f-4ea9-9eac-9e9a92715690" xlink:to="loc_us-gaap_CommonClassAMember_9c8ac3c8-e6a4-469d-b04d-4dc24829ac77" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassBMember_e241c679-e66b-4a19-8c2b-b8d4a442c37b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CommonClassBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_b5044cef-9f1f-4ea9-9eac-9e9a92715690" xlink:to="loc_us-gaap_CommonClassBMember_e241c679-e66b-4a19-8c2b-b8d4a442c37b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/OTHERITEMSANDCHARGES" xlink:type="simple" xlink:href="cwst-20200630.xsd#OTHERITEMSANDCHARGES"/>
  <link:definitionLink xlink:role="http://casella.com/role/OTHERITEMSANDCHARGES" xlink:type="extended" id="ie15e58e643ed4838be7f51a917682c97_OTHERITEMSANDCHARGES"/>
  <link:roleRef roleURI="http://casella.com/role/OTHERITEMSANDCHARGESDetails" xlink:type="simple" xlink:href="cwst-20200630.xsd#OTHERITEMSANDCHARGESDetails"/>
  <link:definitionLink xlink:role="http://casella.com/role/OTHERITEMSANDCHARGESDetails" xlink:type="extended" id="i93c35d5b940442d79a7ecf9684f4a7da_OTHERITEMSANDCHARGESDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SiteContingencyLineItems_e1d35cb5-ce96-4d54-bc64-549ec1fac518" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SiteContingencyLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ExpenseFromAcquisitionActivities_b71fb212-5299-4974-beaa-6ba5440782f9" xlink:href="cwst-20200630.xsd#cwst_ExpenseFromAcquisitionActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SiteContingencyLineItems_e1d35cb5-ce96-4d54-bc64-549ec1fac518" xlink:to="loc_cwst_ExpenseFromAcquisitionActivities_b71fb212-5299-4974-beaa-6ba5440782f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SouthbridgeLandfillClosureCharge_9d0c12d6-9268-4c17-9893-5a36ca08d4b3" xlink:href="cwst-20200630.xsd#cwst_SouthbridgeLandfillClosureCharge"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SiteContingencyLineItems_e1d35cb5-ce96-4d54-bc64-549ec1fac518" xlink:to="loc_cwst_SouthbridgeLandfillClosureCharge_9d0c12d6-9268-4c17-9893-5a36ca08d4b3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LegalFees_3188f8e0-c354-40ba-8215-a4d21dfe8654" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LegalFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SiteContingencyLineItems_e1d35cb5-ce96-4d54-bc64-549ec1fac518" xlink:to="loc_us-gaap_LegalFees_3188f8e0-c354-40ba-8215-a4d21dfe8654" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationExpense_e033d8e7-de94-4406-a1f0-66504b5fdbb6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EnvironmentalRemediationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SiteContingencyLineItems_e1d35cb5-ce96-4d54-bc64-549ec1fac518" xlink:to="loc_us-gaap_EnvironmentalRemediationExpense_e033d8e7-de94-4406-a1f0-66504b5fdbb6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalCostsRecognizedRecoveryCreditedToExpense_d597a6e3-1f18-459f-9c94-ec3782aa5a9c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EnvironmentalCostsRecognizedRecoveryCreditedToExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SiteContingencyLineItems_e1d35cb5-ce96-4d54-bc64-549ec1fac518" xlink:to="loc_us-gaap_EnvironmentalCostsRecognizedRecoveryCreditedToExpense_d597a6e3-1f18-459f-9c94-ec3782aa5a9c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SiteContingencyTable_87b55f04-48cf-46fd-b278-30c0bc9d0bdf" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SiteContingencyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SiteContingencyLineItems_e1d35cb5-ce96-4d54-bc64-549ec1fac518" xlink:to="loc_us-gaap_SiteContingencyTable_87b55f04-48cf-46fd-b278-30c0bc9d0bdf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationSiteAxis_0c6a2014-2f96-4952-a212-cbaf5deee7ce" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EnvironmentalRemediationSiteAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SiteContingencyTable_87b55f04-48cf-46fd-b278-30c0bc9d0bdf" xlink:to="loc_us-gaap_EnvironmentalRemediationSiteAxis_0c6a2014-2f96-4952-a212-cbaf5deee7ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationSiteDomain_0c6a2014-2f96-4952-a212-cbaf5deee7ce_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EnvironmentalRemediationSiteDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteAxis_0c6a2014-2f96-4952-a212-cbaf5deee7ce" xlink:to="loc_us-gaap_EnvironmentalRemediationSiteDomain_0c6a2014-2f96-4952-a212-cbaf5deee7ce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EnvironmentalRemediationSiteDomain_1fdfe056-e758-4ee4-a74b-0a4e5935acb7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EnvironmentalRemediationSiteDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteAxis_0c6a2014-2f96-4952-a212-cbaf5deee7ce" xlink:to="loc_us-gaap_EnvironmentalRemediationSiteDomain_1fdfe056-e758-4ee4-a74b-0a4e5935acb7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SouthbridgeLandfillMember_8678b504-e98c-4fab-be11-4696bd9bd90d" xlink:href="cwst-20200630.xsd#cwst_SouthbridgeLandfillMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EnvironmentalRemediationSiteDomain_1fdfe056-e758-4ee4-a74b-0a4e5935acb7" xlink:to="loc_cwst_SouthbridgeLandfillMember_8678b504-e98c-4fab-be11-4696bd9bd90d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTS" xlink:type="simple" xlink:href="cwst-20200630.xsd#FAIRVALUEOFFINANCIALINSTRUMENTS"/>
  <link:definitionLink xlink:role="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTS" xlink:type="extended" id="i03e4b8d21a5540d49f802c211d786376_FAIRVALUEOFFINANCIALINSTRUMENTS"/>
  <link:roleRef roleURI="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSTables" xlink:type="simple" xlink:href="cwst-20200630.xsd#FAIRVALUEOFFINANCIALINSTRUMENTSTables"/>
  <link:definitionLink xlink:role="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSTables" xlink:type="extended" id="ib947e7072382434b99f03b60875398c8_FAIRVALUEOFFINANCIALINSTRUMENTSTables"/>
  <link:roleRef roleURI="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSRecurringFairValueMeasurementsDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#FAIRVALUEOFFINANCIALINSTRUMENTSRecurringFairValueMeasurementsDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSRecurringFairValueMeasurementsDetail" xlink:type="extended" id="if317f27d234a4caab9bf1c12dbffa7c8_FAIRVALUEOFFINANCIALINSTRUMENTSRecurringFairValueMeasurementsDetail">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_b0f01f55-5174-4054-bc70-33ed0aace329" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_98d7dd97-1a51-491e-a545-a55d5cdd995c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_b0f01f55-5174-4054-bc70-33ed0aace329" xlink:to="loc_us-gaap_AssetsAbstract_98d7dd97-1a51-491e-a545-a55d5cdd995c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_f101e24f-892f-4a68-be8f-e449ef5a7f49" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_RestrictedCashAndInvestmentsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_98d7dd97-1a51-491e-a545-a55d5cdd995c" xlink:to="loc_us-gaap_RestrictedCashAndInvestmentsNoncurrent_f101e24f-892f-4a68-be8f-e449ef5a7f49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_8128dda7-ead4-44fd-b1aa-121ba43f5af5" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_b0f01f55-5174-4054-bc70-33ed0aace329" xlink:to="loc_us-gaap_LiabilitiesAbstract_8128dda7-ead4-44fd-b1aa-121ba43f5af5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestRateDerivativeLiabilitiesAtFairValue_1b6a3df4-3b29-4c93-a2b3-53e1e740c67d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_InterestRateDerivativeLiabilitiesAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_8128dda7-ead4-44fd-b1aa-121ba43f5af5" xlink:to="loc_us-gaap_InterestRateDerivativeLiabilitiesAtFairValue_1b6a3df4-3b29-4c93-a2b3-53e1e740c67d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0d98d33e-1174-488a-a31e-827c4690070a" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_b0f01f55-5174-4054-bc70-33ed0aace329" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0d98d33e-1174-488a-a31e-827c4690070a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_420627e8-6683-4016-9f88-d4a36905908b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0d98d33e-1174-488a-a31e-827c4690070a" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_420627e8-6683-4016-9f88-d4a36905908b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_420627e8-6683-4016-9f88-d4a36905908b_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_420627e8-6683-4016-9f88-d4a36905908b" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_420627e8-6683-4016-9f88-d4a36905908b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_42e52399-cc3d-4eae-8972-9d9b3475ad05" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_420627e8-6683-4016-9f88-d4a36905908b" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_42e52399-cc3d-4eae-8972-9d9b3475ad05" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_5cebe002-791f-4dea-b57d-0ea7a1ebcf7f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_42e52399-cc3d-4eae-8972-9d9b3475ad05" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_5cebe002-791f-4dea-b57d-0ea7a1ebcf7f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6b875656-576a-4365-8c6c-96203091a8c1" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0d98d33e-1174-488a-a31e-827c4690070a" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6b875656-576a-4365-8c6c-96203091a8c1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_6b875656-576a-4365-8c6c-96203091a8c1_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6b875656-576a-4365-8c6c-96203091a8c1" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_6b875656-576a-4365-8c6c-96203091a8c1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_08c07d26-89f5-453d-a457-a54811a07d8f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6b875656-576a-4365-8c6c-96203091a8c1" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_08c07d26-89f5-453d-a457-a54811a07d8f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_a35b26eb-cdd6-4f9f-a7e7-bfcd951e40d0" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_08c07d26-89f5-453d-a457-a54811a07d8f" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_a35b26eb-cdd6-4f9f-a7e7-bfcd951e40d0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_76269ae7-4787-433c-afe9-c46c312f3bae" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_08c07d26-89f5-453d-a457-a54811a07d8f" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_76269ae7-4787-433c-afe9-c46c312f3bae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_d5ce97ff-9f98-473a-ba5b-0ecfacdd6144" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_08c07d26-89f5-453d-a457-a54811a07d8f" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_d5ce97ff-9f98-473a-ba5b-0ecfacdd6144" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_c1d46d5d-cf15-405a-992e-49ad4eab7263" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0d98d33e-1174-488a-a31e-827c4690070a" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_c1d46d5d-cf15-405a-992e-49ad4eab7263" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_c1d46d5d-cf15-405a-992e-49ad4eab7263_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_c1d46d5d-cf15-405a-992e-49ad4eab7263" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_c1d46d5d-cf15-405a-992e-49ad4eab7263_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_f875a724-b125-4e50-bef0-b5dcb637c48f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_c1d46d5d-cf15-405a-992e-49ad4eab7263" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_f875a724-b125-4e50-bef0-b5dcb637c48f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandfillMember_9ef78c13-bccf-4cbe-88f2-f38ab6be0ca3" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LandfillMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_f875a724-b125-4e50-bef0-b5dcb637c48f" xlink:to="loc_us-gaap_LandfillMember_9ef78c13-bccf-4cbe-88f2-f38ab6be0ca3" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSAdditionalInformationDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#FAIRVALUEOFFINANCIALINSTRUMENTSAdditionalInformationDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/FAIRVALUEOFFINANCIALINSTRUMENTSAdditionalInformationDetail" xlink:type="extended" id="ieb69d5ca19a5460a8ca14ad31921d68a_FAIRVALUEOFFINANCIALINSTRUMENTSAdditionalInformationDetail">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_02527099-6c16-4d3d-bb8a-ce0cb3b7cf3f" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFairValue_03e436d7-6778-4933-92b2-409cf0313982" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_02527099-6c16-4d3d-bb8a-ce0cb3b7cf3f" xlink:to="loc_us-gaap_DebtInstrumentFairValue_03e436d7-6778-4933-92b2-409cf0313982" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LinesOfCreditFairValueDisclosure_659060f3-77eb-4d91-82ff-0ecb221844db" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LinesOfCreditFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_02527099-6c16-4d3d-bb8a-ce0cb3b7cf3f" xlink:to="loc_us-gaap_LinesOfCreditFairValueDisclosure_659060f3-77eb-4d91-82ff-0ecb221844db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_b438c403-b0fc-4597-8d25-6c37a1e3e2da" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_02527099-6c16-4d3d-bb8a-ce0cb3b7cf3f" xlink:to="loc_us-gaap_DebtInstrumentTable_b438c403-b0fc-4597-8d25-6c37a1e3e2da" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_060c6db4-2b84-4cb2-bc49-3905e1ec71aa" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_b438c403-b0fc-4597-8d25-6c37a1e3e2da" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_060c6db4-2b84-4cb2-bc49-3905e1ec71aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_060c6db4-2b84-4cb2-bc49-3905e1ec71aa_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_060c6db4-2b84-4cb2-bc49-3905e1ec71aa" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_060c6db4-2b84-4cb2-bc49-3905e1ec71aa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_607b7c27-7274-4d32-9698-e59f94496e79" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_060c6db4-2b84-4cb2-bc49-3905e1ec71aa" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_607b7c27-7274-4d32-9698-e59f94496e79" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_565183c8-46f4-4dbc-9149-0ecd25d823af" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_607b7c27-7274-4d32-9698-e59f94496e79" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_565183c8-46f4-4dbc-9149-0ecd25d823af" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_07a53a96-c027-43d6-a537-0e13d58a7d35" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_565183c8-46f4-4dbc-9149-0ecd25d823af" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_07a53a96-c027-43d6-a537-0e13d58a7d35" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_746328ab-cd2b-44db-a5d1-cc1af3795cbf" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_565183c8-46f4-4dbc-9149-0ecd25d823af" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_746328ab-cd2b-44db-a5d1-cc1af3795cbf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_9ab2d4b9-2d26-4fbc-8299-72202af3cd47" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_b438c403-b0fc-4597-8d25-6c37a1e3e2da" xlink:to="loc_us-gaap_DebtInstrumentAxis_9ab2d4b9-2d26-4fbc-8299-72202af3cd47" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_9ab2d4b9-2d26-4fbc-8299-72202af3cd47_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_9ab2d4b9-2d26-4fbc-8299-72202af3cd47" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_9ab2d4b9-2d26-4fbc-8299-72202af3cd47_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_2c9bbd01-19cf-4a0c-bd3f-a079d89d61db" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_9ab2d4b9-2d26-4fbc-8299-72202af3cd47" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_2c9bbd01-19cf-4a0c-bd3f-a079d89d61db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_TermFacilitydueOctober2023TermLoanBFacilityMember_920bf48a-aec4-4d58-89fe-b965c8d6c79d" xlink:href="cwst-20200630.xsd#cwst_TermFacilitydueOctober2023TermLoanBFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_2c9bbd01-19cf-4a0c-bd3f-a079d89d61db" xlink:to="loc_cwst_TermFacilitydueOctober2023TermLoanBFacilityMember_920bf48a-aec4-4d58-89fe-b965c8d6c79d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_b939c544-e8b6-4488-8351-675c67954ed6" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_b438c403-b0fc-4597-8d25-6c37a1e3e2da" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_b939c544-e8b6-4488-8351-675c67954ed6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_b939c544-e8b6-4488-8351-675c67954ed6_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_b939c544-e8b6-4488-8351-675c67954ed6" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_b939c544-e8b6-4488-8351-675c67954ed6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_9222b1c8-9daa-4a4d-a707-17b591b198bd" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_b939c544-e8b6-4488-8351-675c67954ed6" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_9222b1c8-9daa-4a4d-a707-17b591b198bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_FixedRateBondsMember_dc3e9ba1-dd51-484e-82b1-21363f145fe5" xlink:href="cwst-20200630.xsd#cwst_FixedRateBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_9222b1c8-9daa-4a4d-a707-17b591b198bd" xlink:to="loc_cwst_FixedRateBondsMember_dc3e9ba1-dd51-484e-82b1-21363f145fe5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_74af274d-8bee-4dd1-8492-a3892ebcfb01" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_9222b1c8-9daa-4a4d-a707-17b591b198bd" xlink:to="loc_us-gaap_SecuredDebtMember_74af274d-8bee-4dd1-8492-a3892ebcfb01" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/SEGMENTREPORTING" xlink:type="simple" xlink:href="cwst-20200630.xsd#SEGMENTREPORTING"/>
  <link:definitionLink xlink:role="http://casella.com/role/SEGMENTREPORTING" xlink:type="extended" id="i023a20d9869647b1bbfc3496ac5fbb87_SEGMENTREPORTING"/>
  <link:roleRef roleURI="http://casella.com/role/SEGMENTREPORTINGTables" xlink:type="simple" xlink:href="cwst-20200630.xsd#SEGMENTREPORTINGTables"/>
  <link:definitionLink xlink:role="http://casella.com/role/SEGMENTREPORTINGTables" xlink:type="extended" id="ic83525f725ec43e3ba4f6c2db6968d4b_SEGMENTREPORTINGTables"/>
  <link:roleRef roleURI="http://casella.com/role/SEGMENTREPORTINGSummaryofFinancialInformationbyReportableSegmentDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#SEGMENTREPORTINGSummaryofFinancialInformationbyReportableSegmentDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/SEGMENTREPORTINGSummaryofFinancialInformationbyReportableSegmentDetail" xlink:type="extended" id="i3ee9c541149a42bb8f77642c40816f66_SEGMENTREPORTINGSummaryofFinancialInformationbyReportableSegmentDetail">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_367f073f-d8da-4e68-85ba-3cbcf3993300" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_77122f75-514a-433e-9816-2806a3e5f068" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_367f073f-d8da-4e68-85ba-3cbcf3993300" xlink:to="loc_us-gaap_Revenues_77122f75-514a-433e-9816-2806a3e5f068" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_614133f1-7192-4711-bf34-b987d7a3030b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_367f073f-d8da-4e68-85ba-3cbcf3993300" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_614133f1-7192-4711-bf34-b987d7a3030b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_157919dc-45e9-4c37-9f29-f64d5b0c4a6b" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_367f073f-d8da-4e68-85ba-3cbcf3993300" xlink:to="loc_us-gaap_OperatingIncomeLoss_157919dc-45e9-4c37-9f29-f64d5b0c4a6b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_59642b09-5c6c-4a22-8921-a03bd31b2ee7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_367f073f-d8da-4e68-85ba-3cbcf3993300" xlink:to="loc_us-gaap_Assets_59642b09-5c6c-4a22-8921-a03bd31b2ee7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_41ccbf1a-99d0-4b64-9d98-2d61bfcaf6ca" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_367f073f-d8da-4e68-85ba-3cbcf3993300" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_41ccbf1a-99d0-4b64-9d98-2d61bfcaf6ca" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsAxis_0f355293-d3da-4331-8331-81a9ee6711e3" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_ConsolidationItemsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_41ccbf1a-99d0-4b64-9d98-2d61bfcaf6ca" xlink:to="loc_srt_ConsolidationItemsAxis_0f355293-d3da-4331-8331-81a9ee6711e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_0f355293-d3da-4331-8331-81a9ee6711e3_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ConsolidationItemsAxis_0f355293-d3da-4331-8331-81a9ee6711e3" xlink:to="loc_srt_ConsolidationItemsDomain_0f355293-d3da-4331-8331-81a9ee6711e3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationItemsDomain_2b57bd42-e061-49c3-b831-4e5a00c5bfb2" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_ConsolidationItemsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ConsolidationItemsAxis_0f355293-d3da-4331-8331-81a9ee6711e3" xlink:to="loc_srt_ConsolidationItemsDomain_2b57bd42-e061-49c3-b831-4e5a00c5bfb2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingSegmentsMember_976a9c22-58d2-4d30-a7ee-34cf76b8a0f7" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_OperatingSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_2b57bd42-e061-49c3-b831-4e5a00c5bfb2" xlink:to="loc_us-gaap_OperatingSegmentsMember_976a9c22-58d2-4d30-a7ee-34cf76b8a0f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ConsolidationEliminationsMember_095f8d76-03a5-4edb-902f-3e999c383369" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_ConsolidationEliminationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ConsolidationItemsDomain_2b57bd42-e061-49c3-b831-4e5a00c5bfb2" xlink:to="loc_srt_ConsolidationEliminationsMember_095f8d76-03a5-4edb-902f-3e999c383369" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_612d671f-b41f-46eb-9fd5-ad6bb0322b54" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_41ccbf1a-99d0-4b64-9d98-2d61bfcaf6ca" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_612d671f-b41f-46eb-9fd5-ad6bb0322b54" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_612d671f-b41f-46eb-9fd5-ad6bb0322b54_default" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_612d671f-b41f-46eb-9fd5-ad6bb0322b54" xlink:to="loc_us-gaap_SegmentDomain_612d671f-b41f-46eb-9fd5-ad6bb0322b54_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_ca971699-87f3-41cf-ae3c-0f21d9b9bb03" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_612d671f-b41f-46eb-9fd5-ad6bb0322b54" xlink:to="loc_us-gaap_SegmentDomain_ca971699-87f3-41cf-ae3c-0f21d9b9bb03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_EasternRegionMember_cc757d8e-5f59-405f-9607-531142f4ec8f" xlink:href="cwst-20200630.xsd#cwst_EasternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_ca971699-87f3-41cf-ae3c-0f21d9b9bb03" xlink:to="loc_cwst_EasternRegionMember_cc757d8e-5f59-405f-9607-531142f4ec8f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_WesternRegionMember_a25284ce-a453-4311-96a3-4e0c8eae69b4" xlink:href="cwst-20200630.xsd#cwst_WesternRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_ca971699-87f3-41cf-ae3c-0f21d9b9bb03" xlink:to="loc_cwst_WesternRegionMember_a25284ce-a453-4311-96a3-4e0c8eae69b4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsRegionMember_f51f7172-ecc2-447c-9c78-66748f91936c" xlink:href="cwst-20200630.xsd#cwst_ResourceSolutionsRegionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_ca971699-87f3-41cf-ae3c-0f21d9b9bb03" xlink:to="loc_cwst_ResourceSolutionsRegionMember_f51f7172-ecc2-447c-9c78-66748f91936c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllOtherSegmentsMember_87d5d7e9-7142-4f69-a67a-6442169dfe5d" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_AllOtherSegmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_ca971699-87f3-41cf-ae3c-0f21d9b9bb03" xlink:to="loc_us-gaap_AllOtherSegmentsMember_87d5d7e9-7142-4f69-a67a-6442169dfe5d" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://casella.com/role/SEGMENTREPORTINGSummaryofRevenuesAttributabletoServicesProvidedbyCompanyDetail" xlink:type="simple" xlink:href="cwst-20200630.xsd#SEGMENTREPORTINGSummaryofRevenuesAttributabletoServicesProvidedbyCompanyDetail"/>
  <link:definitionLink xlink:role="http://casella.com/role/SEGMENTREPORTINGSummaryofRevenuesAttributabletoServicesProvidedbyCompanyDetail" xlink:type="extended" id="i74099def6a8b4108ae2f7618b04cbb61_SEGMENTREPORTINGSummaryofRevenuesAttributabletoServicesProvidedbyCompanyDetail">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_d29019ad-8e6b-4917-9fc3-52920c18da6e" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Revenues_aa3f6942-4c54-4b98-8780-10882f95376c" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_Revenues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_d29019ad-8e6b-4917-9fc3-52920c18da6e" xlink:to="loc_us-gaap_Revenues_aa3f6942-4c54-4b98-8780-10882f95376c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_10f5d085-8885-4945-9916-bb28256ce4f8" xlink:href="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_d29019ad-8e6b-4917-9fc3-52920c18da6e" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_10f5d085-8885-4945-9916-bb28256ce4f8" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_ba65405e-d85b-47bf-9f64-533a31a70fc3" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_10f5d085-8885-4945-9916-bb28256ce4f8" xlink:to="loc_srt_ProductOrServiceAxis_ba65405e-d85b-47bf-9f64-533a31a70fc3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_ba65405e-d85b-47bf-9f64-533a31a70fc3_default" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_ba65405e-d85b-47bf-9f64-533a31a70fc3" xlink:to="loc_srt_ProductsAndServicesDomain_ba65405e-d85b-47bf-9f64-533a31a70fc3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_5a9d4e65-4761-41a6-a55d-77b0689b900d" xlink:href="http://xbrl.fasb.org/srt/2020/elts/srt-2020-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_ba65405e-d85b-47bf-9f64-533a31a70fc3" xlink:to="loc_srt_ProductsAndServicesDomain_5a9d4e65-4761-41a6-a55d-77b0689b900d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsMember_41a747b5-dae1-4d02-8202-c9608589194e" xlink:href="cwst-20200630.xsd#cwst_SolidWasteOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_5a9d4e65-4761-41a6-a55d-77b0689b900d" xlink:to="loc_cwst_SolidWasteOperationsMember_41a747b5-dae1-4d02-8202-c9608589194e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsCollectionMember_4a1b434f-643e-4362-8ded-c2b45062c650" xlink:href="cwst-20200630.xsd#cwst_SolidWasteOperationsCollectionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_41a747b5-dae1-4d02-8202-c9608589194e" xlink:to="loc_cwst_SolidWasteOperationsCollectionMember_4a1b434f-643e-4362-8ded-c2b45062c650" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsDisposalMember_036a4d8c-b664-47e9-b45b-8b799ead4fa5" xlink:href="cwst-20200630.xsd#cwst_SolidWasteOperationsDisposalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_41a747b5-dae1-4d02-8202-c9608589194e" xlink:to="loc_cwst_SolidWasteOperationsDisposalMember_036a4d8c-b664-47e9-b45b-8b799ead4fa5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsPowerGenerationMember_f3469e4d-86c6-4987-9c3b-6a0c7b543eba" xlink:href="cwst-20200630.xsd#cwst_SolidWasteOperationsPowerGenerationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_41a747b5-dae1-4d02-8202-c9608589194e" xlink:to="loc_cwst_SolidWasteOperationsPowerGenerationMember_f3469e4d-86c6-4987-9c3b-6a0c7b543eba" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_SolidWasteOperationsProcessingMember_916391e1-13b0-462b-8db2-bb5f616a0c1f" xlink:href="cwst-20200630.xsd#cwst_SolidWasteOperationsProcessingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_SolidWasteOperationsMember_41a747b5-dae1-4d02-8202-c9608589194e" xlink:to="loc_cwst_SolidWasteOperationsProcessingMember_916391e1-13b0-462b-8db2-bb5f616a0c1f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_ResourceSolutionsOperationsMember_47cf7c46-41d8-48c4-9d37-cf3f522c7991" xlink:href="cwst-20200630.xsd#cwst_ResourceSolutionsOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_5a9d4e65-4761-41a6-a55d-77b0689b900d" xlink:to="loc_cwst_ResourceSolutionsOperationsMember_47cf7c46-41d8-48c4-9d37-cf3f522c7991" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_OrganicsMember_c5294473-8b20-4c5b-a20e-20c817b1ca3c" xlink:href="cwst-20200630.xsd#cwst_OrganicsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_ResourceSolutionsOperationsMember_47cf7c46-41d8-48c4-9d37-cf3f522c7991" xlink:to="loc_cwst_OrganicsMember_c5294473-8b20-4c5b-a20e-20c817b1ca3c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_CustomerSolutionsMember_bfd9a554-bcfc-48cd-bda7-25ae228ed0b9" xlink:href="cwst-20200630.xsd#cwst_CustomerSolutionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_ResourceSolutionsOperationsMember_47cf7c46-41d8-48c4-9d37-cf3f522c7991" xlink:to="loc_cwst_CustomerSolutionsMember_bfd9a554-bcfc-48cd-bda7-25ae228ed0b9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_cwst_RecyclingRevenuesMember_60d54b0d-6aaa-4c2a-bb89-2038efed2809" xlink:href="cwst-20200630.xsd#cwst_RecyclingRevenuesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_cwst_ResourceSolutionsOperationsMember_47cf7c46-41d8-48c4-9d37-cf3f522c7991" xlink:to="loc_cwst_RecyclingRevenuesMember_60d54b0d-6aaa-4c2a-bb89-2038efed2809" xlink:type="arc" order="3"/>
  </link:definitionLink>
</link:linkbase>
