Outreach is a treatment modality for engaging underserved populations in health care. Nowhere is outreach more relevant than in delivering services to homeless persons with mental illness. Programs providing outreach to homeless people have been in existence for at least two decades and a craft has developed naturalistically. However, there has been insufficient formal examination of factors that influence the effectiveness of outreach and how it is actually performed. The authors present an in-depth examination of issues related to outreach to the homeless. They review different outreach modalities, the role of the individual clinician, and the art of teamwork. They also discuss external issues such as financing, access to housing, interactions with other professions, and working conditions. The authors conclude with a brief discussion concerning the application of outreach to populations other than homeless individuals with psychiatric disorders and suggest future directions for improving our understanding of this important modality. (Authors)