This article describes a public health approach to the study of service systems for people with substance use disorders. Such an approach is broadly conceived to include the consideration not only of specialized services for alcohol and drug dependence, but also of medical care and social welfare services that interact with and complement specialized drug and alcohol services. After describing a conceptual model of the substance abuse service system, we discuss how systems of care may contribute to population health and social welfare by reducing the societal burden of substance use disorders. The article then summarizes key systems issues pertaining to mediators and moderators of effective treatment systems. The potential benefits of systems thinking in relation to the management of substance use disorders are described. It is suggested that systems concepts and research may help to improve access, efficiency, economy, continuity of care, and effectiveness, thereby improving the population impact of treatment services.