In order to more effectively treat substance use disorders in persons with severe mental illness, integrated treatment models have been developed for these ‘co-occurring disorders.’ The nature of clinical services provided in these programs is described in this article, with brief attention to research on these programs. Clinical services are generally guided by a stage model of change, in which the client's motivation to change (or stage of treatment: engagement, persuasion, active treatment, relapse prevention) informs the optimal selection of appropriate interventions. Psychotherapeutic services for co-occurring disorders include a range of individual (motivational interviewing, cognitive-behavioral counseling), group (stage-wise groups, social skills training, self-help), and family (single family, multifamily formats) interventions. Supplementary interventions are also commonly provided, including psychopharmacological, residential, coerced and involuntary, and vocational rehabilitation. Access to a range of different interventions for co-occurring disorders may improve the ability of clinicians to engage and tailor treatment to meet clients' unique needs. The research base supporting integrated treatment for co-occurring disorders is limited at this time owing to the small number of methodologically rigorous studies from which to draw conclusions. Nevertheless, the available evidence suggests that integrated treatment programs improve outcomes compared with nonintegrated approaches, and that more research is needed to evaluate the benefits of these programs.