Housing First is an effective intervention that ends and prevents homelessness for individuals with severe mental illness and co-occurring addictions. By providing permanent, independent housing without prerequisites for sobriety and treatment, and by offering support services through consumer-driven Assertive Community Treatment teams, Housing First removes some of the major obstacles to obtaining and maintaining housing for consumers who are chronically homeless. In this study, consumers diagnosed with severe mental illness and who had the longest histories of shelter use in a suburban county were randomly assigned to either one of two Housing First programs or to a treatment-as-usual control group. Participants assigned to Housing First were placed in permanent housing at higher rates than the treatment-as-usual group and, over the course of four years, the majority of consumers placed by both Housing First agencies were able to maintain permanent, independent housing. Results also highlight that providers new to Housing First must be aware of ways in which their practices may deviate from the essential features of Housing First, particularly with respect to enrolling eligible consumers on a first-come, first-served basis and separating clinical issues from tenant or housing responsibilities. Finally, other aspects of successfully implementing a Housing First program are discussed.