In Great Britain 1–2 million people may be homeless. Most homeless people are men, but about 10–25% are women, of whom about half are accompanied by children. Significant mental illness is present in 30–50% of the homeless: functional psychoses predominate; acute distress and personality dysfunction are also prevalent. Co-morbidity of mental illness and substance abuse occurs in 20%, and physical morbidity rates exceed those of domiciled populations. The homeless mentally ill also have many social needs. Pathways to homelessness are complex; deinstitutionalisation may be only one possible cause of the increase in the number of homeless people. There is much recent research estimating the extent of mental illness and the characteristics of selected subgroups of accessible homeless people. The evaluation of potential service solutions has received less attention. This review outlines the research, highlights current views on the definition and classification of homeless populations, and offers some guidelines on avenues which need to be explored.