This article describes recent research on the prevalence of alcohol, drug, and mental (ADM) disorders and the characteristics of homeless substance abusers and persons with mental illness. Methodological problems in homelessness research are reviewed, particularly in relation to definitions of homelessness and sampling- and case-ascertainment methods. Prevalence rates of ADM disorders are much higher in homeless groups than in the general population. As is true of homeless people in general, homeless substance abusers and mentally ill persons are characterized by extreme poverty; underutilization of public entitlements; isolation from family, friends, and other support networks; frequent contact with correctional agencies; and poor general health. Knowledge of these disadvantages should be used to advocate for better services to prevent homelessness and support homeless people.