Health and social services integration is particularly relevant for populations whose needs span physical health, mental health, housing, and disability services, along with others. Veterans, homeless, chronically ill, and aging are among those populations. This review examines recent peer-reviewed literature about different approaches to services integration, rationales behind those approaches, and successes of those approaches, including factors that make them succeed or fail. The focus here is on services that cross disciplinary boundaries; that is, those that integrate health services with social services, health services with mental health services, or one social service with a categorically different social service.