Consultations with community-based service providers in Toronto identified a lack of strong research evidence about successful community-based interventions that address the needs of homeless clients experiencing concurrent mental health and substance use disorders. We undertook a collaborative research effort between academic-based and community-based partners to conduct a systematic evidence synthesis drawing heavily from Pawson's realist review methodology to focus on both whether programs are successful and why and how they lead to improved outcomes. We examined scholarly and nonscholarly literature to explore program approaches and program elements that lead to improvements in mental health and substance use disorders among homeless individuals with concurrent disorders (CD). Information related to program contexts, elements, and successes and failures were extracted and further supplemented by key informant interviews and author communication regarding reviewed published studies. From the ten programs that we reviewed, we identified six important and promising program strategies that reduce mental health and, to a far lesser degree, substance use problems: client choice in treatment decision-making, positive interpersonal relationships between client and provider, assertive community treatment approaches, providing supportive housing, providing supports for instrumental needs, and nonrestrictive program approaches. These promising program strategies function, in part, by promoting and supporting autonomy among homeless adults experiencing CD. Our realist informed review is a useful methodology for synthesizing complex programming information on community-based interventions.