The term ‘homeless’ has been used to describe populations as diverse as those sleeping in the shelter of a cardboard box, to those sleeping on a friend’s floor. Given such wide definition, it is not surprising that estimates of the numbers involved vary greatly from survey to survey and from one country to another. But regardless of the definition, there is consensus that the numbers of homeless people in most Western urban areas increased during the past two decades, reflecting a scarcity of low-cost housing, the erosion of traditional family networks, and downsizing in the organization and delivery of supportive services. Of all these factors, the shortage of affordable accommodation is the most important. For example, in England there has been a 40 per cent increase since 2002 in the number of households on waiting lists for social housing with estimates that a minimum of 20 000 housing units above current government targets are required to simply meet newly arising urgent need.⁽¹⁾

Compared with a domiciled population, homeless people are less likely to have completed basic education, less likely to have ever held employment, and more likely to have experienced parental neglect and abuse in their childhood.⁽²⁾

Given the evidence linking homelessness to poverty and social disadvantage, it is hardly surprising that homeless people report higher rates of psychiatric disorder relative to the general population. While rates vary depending on the particular measure of mental illness adopted by each study and by the homeless population being investigated, most report major psychiatric disorder in 30 to 60 per cent of those using emergency shelters and sleeping rough. The prevalence of schizophrenia and other psychoses is particularly high amongst the middle-aged residents of long-stay hostels, while depression, generalized anxiety, and impulsive self-harm are more typically encountered in younger runaways and adolescent populations. Alcoholism and drug dependency are present in as many as two-thirds of men and a third of homeless women. Co-morbidity of mental illness and substance use disorder is the rule rather than the exception as are the co-occurrence of respiratory disease, infections, trauma, and the physical consequences of poor diet, poor hygiene, and the complications of substance abuse. Of growing concern is the accumulation of older multiply disabled populations in some North American cities.⁽³⁾

The typical pattern of service utilization of the severely mentally ill among the homeless population is one of extremes—bursts of involuntary hospital admissions and compulsory treatment interspersed with long periods of neglect and isolation. Many of those who are found sleeping rough or resident in temporary shelters have found their way to these locations as a conscious effort to avoid contact with health and social care professionals and remain unwilling to be part of any structured rehabilitation programme.