Much of the hot water in multifamily buildings is provided by natural gas and other fossil fuels, which emit harmful greenhouse gases (GHGs). Utilities and policymakers are increasingly looking to technologies such as electric heat pump water heaters (HPWHs) to help cut these emissions, while providing other benefits such as grid flexibility. This study from New Buildings Institute was developed to help stakeholders understand the economics and potential benefits of having fossil fuel water heaters in multifamily buildings retrofit with HPWHs, as well as the types of program and policy interventions that could be used to transform the market.