After 20 years of development and research, dual diagnosis services for clients with severe mental illness are emerging as an evidence-based practice. Effective dual diagnosis programs combine mental health and substance abuse interventions that are tailored for the complex needs of clients with comorbid disorders. The authors describe the critical components of effective programs, which include a comprehensive, long-term, staged approach to recovery; assertive outreach; motivational interventions; provision of help to clients in acquiring skills and supports to manage both illnesses and to pursue functional goals; and cultural sensitivity and competence. Many state mental health systems are implementing dual diagnosis services, but high-quality services are rare. The authors provide an overview of the numerous barriers to implementation and describe implementation strategies to overcome the barriers. Current approaches to implementing dual diagnosis programs involve organizational and financing changes at the policy level, clarity of program mission with structural changes to support dual diagnosis services, training and supervision for clinicians, and dissemination of accurate information to consumers and families to support understanding, demand, and advocacy.