Drake RE, Morse G, Brunette MF, Torrey WC. Evolving U.S. service model for patients with severe mental illness and co-occurring substance use disorder. Co-occurring severe mental illness and substance use disorder has been recognized as a common problem in the U.S. since the early 1980s (1–3). For these individuals with co-occurring disorders, research demonstrates the effectiveness of various forms of combining, blending, or integrating mental health and substance abuse treatments ( 4). The evolving U.S. service model for integrated dual disorders treatment emphasizes several key elements: implementation, leadership, training, engagement, assessment, counseling for all patients, ancillary treatments for those with multiple needs, secondary treatments for patients who are nonresponders, and quality assurance regarding process and outcomes.