Patients with severe mental disorders such as schizophrenia and co-occurring substance use disorders traditionally received treatments for their two disorders from two different sets of clinicians in parallel treatment systems. Dissatisfaction with this clinical tradition led to the development of integrated treatment models in which the same clinicians or teams of clinicians provide substance abuse treatment and mental health treatment in a coordinated fashion. We reviewed 36 research studies on the effectiveness of integrated treatment for dually diagnosed patients. Studies of adding dual-disorders groups to traditional services, studies of intensive integrated treatments in controlled settings, and studies of demonstration projects have thus far yielded disappointing results. On the other hand, 10 recent studies of comprehensive, integrated outpatient treatment programs provide encouraging evidence of the programs' potential to engage dually diagnosed patients in services and to help them reduce substance abuse and attain remission. Outcomes related to hospital use, psychiatric symptoms, and other domains are less consistent. Several program features appear to be associated with effectiveness: assertive outreach, case management, and a longitudinal, stage-wise, motivational approach to substance abuse treatment. Given the magnitude and severity of the problem of dual disorders, more controlled research on integrated treatment is needed.