Homelessness is lacking stable and appropriate housing. People can be categorized as homeless if they are: living on the streets (primary homelessness); moving between temporary shelters, including houses of friends, family and emergency accommodation (secondary homelessness); living in private boarding houses without a private bathroom or security of tenure (tertiary homelessness).[1] The legal definition of homeless varies from country to country, or among different jurisdictions in the same country or region.[2] United States government homeless enumeration studies[3][4] also include people who sleep in a public or private place not designed for use as a regular sleeping accommodation for human beings.[5][6] People who are homeless are most often unable to acquire and maintain regular, safe, secure and adequate housing due to income that is inconsistent or lacking altogether. Homelessness and poverty are interrelated.[1] There is no methodological consensus on counting the homeless and identifying their needs; therefore in most cities only estimated homeless populations are known.[7]

Homeless family sleeping in the streets of Kolkata, India (top); a homeless man in Paris, France (bottom).

In 2005, an estimated 100 million people worldwide were homeless and as many as one billion people (one in 6.5 at the time) live as squatters, refugees or in temporary shelter, all lacking adequate housing.[8][9][10] Historically in the Western countries, the majority of homeless have been men (50–80%), with single males particularly over represented.[11][12][13]

Homeless child and adult living on the streets

When compared to the general population, people who are homeless experience higher rates of adverse physical and mental health outcomes. Chronic disease severity, respiratory conditions, rates of mental health illnesses and substance use are all often greater in homeless populations than the general population.[14][15] Homelessness is also associated with a high risk of suicide attempts.[16] People experiencing homelessness have limited access to resources and are often disengaged from health services, making them that much more susceptible to extreme weather events (e.g., extreme cold or heat) and ozone levels. These disparities often result in increased morbidity and mortality in the homeless population.

There are a number of organizations that provide help for homeless people.[17] Most countries provide a variety of services to assist homeless people. These services often provide food, shelter (beds), and clothing and may be organized and run by community organizations (often with the help of volunteers) or by government departments or agencies. These programs may be supported by the government, charities, churches, and individual donors. Many cities also have street newspapers, which are publications designed to provide employment opportunities to homeless people. While some homeless people have jobs, some must seek other methods to make a living. Begging or panhandling is one option, but is becoming increasingly illegal in many cities. People who are homeless may have additional conditions, such as physical or mental health issues or substance addiction; these issues make resolving homelessness a challenging policy issue.