Persons who are homeless often have multiple and complex physical, mental, alcohol, and substance use conditions that strongly indicate the need for supportive services to help them regain lives in the community. Mayors of 25 U.S. cities report that substance abuse, lack of affordable housing, and mental illness are the top three causes of homelessness (U.S. Conference of Mayors, 2008). A 2008 U.S. Department of Housing and Urban Development (HUD) assessment finds that • 26.3% of homeless persons are seriously mentally ill , and • 36.5% of homeless persons are chronic substance abusers (HUD, 2009). The comprehensive data sets compiled by HUD do not report specifically on co-occurring mental health and substance use disorders. Further, HUD’s data on mental illness and substance abuse is limited to sheltered persons, and many shelters and transitional housing programs exclude people who are actively using substances. However, a number of sources suggest that about half of adults who are homeless and have a mental illness also have substance abuse issues. For example, the Metro Denver Homeless Initiative (2009) reported that 28 percent of homeless individuals and heads of households have a mental illness, and that half of these (14 percent) have a co-occurring substance use disorder. A compilation of State data submitted to SAMHSA in conjunction with Projects for Assistance in Transition from Homelessness (PATH, 2009), which funds services to people who are homeless and have a mental illness, reveals that 60 percent of PATH clients also have a co-occurring substance use disorder.