People who are dually diagnosed with severe mental illness and substance use disorders constitute 10%-20% of homeless persons. They are a heterogeneous and extremely vulnerable subgroup with complex, poorly understood needs. In this article recent research on the epidemiology, subject characteristics, and service needs of the dually diagnosed homeless population is reviewed. Also, the range of evolving approaches to providing social services, housing, and mental health and substance-abuse treatments; the relevant system issues and legal issues; and problems with current research, as well as future research directions, are discussed. The importance of the distinction between providing appropriate living environments and mental health treatments emerges throughout.