In response to a request from the Paris Prefecture and the Paris City Hall, the objectives of the Samenta study (Mental Health and Addictions Among the Homeless in the Île-de-France Region) were, among other things, to estimate the prevalence of major psychiatric disorders and addictions among people without a permanent residence in the Île-de-France region. The survey methodology was based on a complex sampling design designed to obtain a random sample of individuals using support services. The questionnaire consisted of a section on mental health and addictions, as well as modules aimed at describing social trajectories and living conditions. Participants were interviewed by a team consisting of a professional interviewer and a clinical psychologist. A psychiatrist was consulted at a later stage based on the clinical information gathered to identify a possible psychiatric disorder. Between February and April 2009, 840 people participated in the survey. It is estimated that one-third of this population suffered from severe psychiatric disorders, namely psychotic disorders, severe mood disorders, or anxiety disorders. This prevalence of severe disorders was higher than in the general population; in particular, it was 10 times higher for psychotic disorders. Substance dependence or regular use of psychoactive substances was 3 to 6 times higher than in the general population. These prevalence rates varied by type of housing (higher in low-threshold facilities) and differed depending on the population’s profile (whether or not they were accompanied by children). These findings support the need to improve screening, psychiatric care, and housing for people experiencing homelessness, taking into account the diversity of this population.