Because the efficacy of behavioral interventions is central to applied psychology, the relative merits of competing approaches to an intervention are important. Many comparative studies examine the differential outcomes of alternative methods of psychotherapy. This paper addresses the issue of impact differences among rival intervention methods by focusing on treatment outcome research that emphasizes the relative (or comparative) efficacy of different psychotherapies. The paper has 4 components. First, it explores the concept of relative efficacy. Second, it reviews the extensive evidence on relative efficacy, which is generally consistent with the null hypothesis. Third, it offers a 3-part explanation of the negative evidence on relative efficacy: (a) a statistical argument about how relative efficacy is bound by a modest upper limit; (b) a research design argument about how relative efficacy studies are confounded by multiple factors, which make it difficult to demonstrate differences in treatment effects; and (c) a theoretical argument about how therapists' contributions to treatment outcomes depend more on their clinical abilities than the therapy methods they implement. The final section of the paper outlines questions for future research.