Like others who are extremely poor, people who experience homelessness are more likely to have mental health and substance abuse disorders than those in the general population. Behavioral health problems can both cause and result from homelessness. They are particularly prevalent among people who are homeless as individuals. Among people experiencing homelessness on a given night, 20 percent reported having a Serious Mental Illness and 17 percent a chronic substance use disorder. Co-morbidities that combine mental illness with substance abuse or chronic medical conditions are common among people with chronic patterns of homelessness.
The federal effort to address chronic homelessness (homelessness among people who have disabilities and are homeless for long periods of time or repeatedly) that began in the 1990s emphasized the development of permanent supportive housing (PSH), with the Substance Abuse and Mental Health Services Administration (SAMHSA) promoting it as a key evidence-based practice. PSH provides subsidized housing and supportive services which generally include behavioral health services provided by state mental health systems and/or funded by Medicaid.
A substantial body of research indicates that providing long-term service-enriched housing with low barriers to entry for people who are homeless and who have serious behavioral health challenges is effective for achieving housing stability and positively impacting recovery, and it may also yield cost savings in other systems, including healthcare, jails, and psychiatric services.
Making appropriate services available to homeless people with behavioral health problems has not been without challenges. Relatively limited funding has been available for treatment of people who have a substance abuse disorder, including those with no co-occurring mental health condition, or for the case management that vulnerable people often need to make sure their housing and the various medical services they need work together.
 CoC Homeless Populations and Subpopulations Reports. U.S. Department of Housing and Urban Development.
 Caton, Carol, Carol Wilkins, and Jaqueline Anderson, “People Who Experience Long-term Homelessness: Characteristics and Interventions,” in Deborah Dennis, Gretchen Locke, and Jill Khadduri, eds., Towards Understanding Homelessness. 2007. U.S. Department of Health and Human Services and U.S. Department of Housing and Urban Development.
 Basu, Anirban, Romina Kee, David Buchanan, and Laura S. Sadowski. Comparative Cost Analysis of Housing and Case Management Program for Chronically Ill Homeless Adults Compared to Usual Care. Health Services Research. 2012; 47(1 Pt2): 523-543; Larimer, Mary E. et al. Health Care and Public Service Use and Costs Before and After Provision of Housing for Chronically Homeless Persons With Severe Alcohol Problems. JAMA. 2009; 301(13): 1349-1357.
 Burt, Martha, Carol Wilkins, and Gretchen Locke. Medicaid and Permanent Supportive Housing for Chronically Homeless Individuals: Emerging Practices from the Field. August 2014. U.S. Department of Health and Human Services, Office of the Assistant Secretary for Planning and Evaluation.