The treatment of individuals with coexisting psychoactive substance abuse and severe psychiatric disorders requires an integration of principles from the mental health and chemical dependency fields. The authors outline a conceptual model for treating dually diagnosed patients that consists of four treatment phases--engagement, persuasion, active (or primary) treatment, and relapse prevention. The components of these phases include case management, group therapy, psychopharmacology, toxicologic screening, detoxification, family involvement, and participation in self-help groups. Due to the high morbidity and mortality associated with dual diagnoses, the authors encourage the development, implementation, and scientific evaluation of integrated treatment models targeted toward this population.