In a study in Western societies, homeless people have a higher prevalence of mental illness when compared to the general population. They also are more likely to suffer from alcoholism and drug dependency. It is estimated that 20–25% of homeless people, compared with 6% of the non-homeless, have severe mental illness. Others estimate that up to one-third of the homeless have a mental illness. In January 2015, the most extensive survey ever undertaken found 564,708 people were homeless on a given night in the United States. Depending on the age group in question and how homelessness is defined, the consensus estimate as of 2014 was that, at minimum, 25% of the American homeless—140,000 individuals—were seriously mentally ill at any given point in time. 45% percent of the homeless—250,000 individuals—had any mental illness. More would be labeled homeless if these were annual counts rather than point-in-time counts. Being chronically homeless also means that people with mental illnesses are more likely to experience catastrophic health crises requiring medical intervention or resulting in institutionalization within the criminal justice system. Majority of the homeless population do not have a mental illness. Although there is no correlation between homelessness and mental health, those who are dealing with homelessness are struggling with psychological and emotional distress. The Substance Abuse and Mental Health Services Administration conducted a study and found that in 2010, 26.2 percent of sheltered homeless people had a severe mental illness.

Homelessness or houselessness – also known as a state of being unhoused or unsheltered – is the condition of lacking stable, safe, and functional housing. The general category includes disparate situations, such as living on the streets, moving between temporary accommodation such as family or friends, living in boarding houses with no security of tenure, and people who leave their domiciles because of civil conflict and are refugees within their country.