Objective: This review critically examines conceptual and methodological issues of outcome research designed to evaluate the efficacy of marital therapy (MT). Behavioural marital therapy (BMT), cognitively orientated marital therapy (COMT), emotionally focused marital therapy (EFMT), and insight-oriented marital therapy (IOMT) have provided evidence for efficacy. Method: The initial literature search involved the use of the MEDLINE and Psychlit CD-ROM data bases. A secondary literature search based on citations in articles discovered in the initial search was also conducted. Three broad categories involving the delineation of the client sample, study design considerations, and determination of treatment effects were addressed to investigate the quality of evidence concerning the efficacy of MT. Results: None of the 4 approaches has proven superiority in efficacy, and the research does not provide evidence for the superiority of any one approach in distinct types of marital discord. There is some evidence to indicate that future outcome research should avoid the use of “waiting list control groups,” since their use appears to be neither ethical nor humane in marital discord research. Conclusions: The authors suggest that the concept of treatment efficiency may be more clinically relevant to outcome research. A gold standard for “efficiency” would suggest that a marital therapy approach should produce subjective and objective improvement in 50% of eligible couples, and this improvement should be maintained in half of these couples at the one-year follow-up assessment. This standard would be comparable to the data on the 4 effective approaches described in this review.