Few studies have assessed adequately the effectiveness of alcohol and drug abuse treatments using an appropriate range of reliable outcome measures, a representative sample of alcohol and drug abuse treatment modalities, or more than one perspective on treatment effectiveness. This article evaluates substance abuse treatment using a sample of 742 patients treated in six programs and evaluated at six-month follow-up. The following three major questions were addressed: (1) do patients improve following treatment; (2) are improvements confined to alcohol or drug use, or are they more pervasive; and (3) are these improvements a result of treatment? The results indicated significant and pervasive improvements in virtually all areas for both alcoholics and drug addicts. Major changes were seen in alcohol and drug use, employment, criminal behavior, and psychological function. Patients undergoing long-term treatment showed greater improvement and better six-month outcomes than those undergoing short-term therapy on 12 of 18 criteria. The data provide evidence for the therapeutic benefit of substance abuse treatments.