This report reviews studies of psychosocial interventions for people with co-occurring substance use disorder and severe mental illness. We identified 45 controlled studies (22 experimental and 23 quasi-experimental) of psychosocial dual diagnosis interventions through several search strategies. Three types of interventions (group counseling, contingency management, and residential dual diagnosis treatment) show consistent positive effects on substance use disorder, whereas other interventions have significant impacts on other areas of adjustment (e.g., case management enhances community tenure and legal interventions increase treatment participation). Current studies are limited by heterogeneity of interventions, participants, methods, outcomes, and measures. Treatment of co-occurring severe mental illness and substance use disorder now has a large but heterogeneous evidence base that nevertheless supports several types of interventions. Future research will need to address methodological standardization, longitudinal perspectives, interventions for subgroups and stages, sequenced interventions, and the changing realities of treatment systems.