Although group therapy is the most prevalent treatment modality for substance use disorders, an up-to-date review of treatment outcome literature does not exist. A search of the literature yielded 24 treatment outcome studies comparing group therapy to other treatment conditions. These studies fell into one of six research design categories: (1) group therapy versus no group therapy; (2) group therapy versus individual therapy; (3) group therapy plus individual therapy versus group therapy alone; (4) group therapy plus individual therapy versus individual therapy alone; (5) group therapy versus another group therapy with different content or theoretical orientation; and (6) more group therapy versus less group therapy. In general, treatment outcome studies did not demonstrate differences between group and individual modalities, and no single type of group therapy reliably demonstrated greater efficacy than others. Unique methodological and logistical hurdles encountered in research on group therapy for substance use disorders, as well as considerations for future research, are also discussed.