The authors reviewed the empirical literature concerning the use of twelve-step programs and treatments by patients with co-occurring substance use disorders and other psychiatric disorders. Strong evidence was found that dually diagnosed individuals (DDI), with the possible exception of those with psychotic disorders, attend twelve-step programs at rates comparable to non-DDI. Twelve-step involvement is consistently associated with improved substance use outcomes. Although there have been numerous clinical trials involving twelve step-oriented interventions for DDI, most of the studies suffered from substantial methodological limitations. More work is needed to determine what kinds of twelve-step treatments and programs are effective for various types of patients and elucidate the mechanisms by which these approaches facilitate recovery.