This toolkit from the city of Boston, MA aims to facilitate the expansion of harm reduction services, and “help build a culture of harm reduction” in the city. It covers principles and core practices of harm reduction; engaging and involving the community, including people who use drugs, neighbors, service providers and city government; and designing, delivering and evaluating harm reduction services and programs. Examples are given of harm reduction services provided in different contexts and settings, including on the street, at a fixed site, in healthcare settings, and in housing programs.