Housing First is an innovative approach to helping individuals with serious mental illness to leave homelessness and begin the process of establishing a meaningful life in the community. The approach has become part of a social movement that is transforming the response to homelessness for this population in Canada, the United States, and western E urope. Curiously, this rapid uptake of Housing First has been based on a small amount of research to date, conducted mostly in large A merican cities. The A t Home / Chez Soi (AHCS) Demonstration Project, launched by the Mental Health Commission of Canada in 2008, was intended to investigate in a rigorous manner the implementation, effectiveness, and cost-effectiveness of Housing First in the Canadian context.