The literature on outcome research in marital therapy is reviewed. Issues considered include the nature of outcome criteria, the need to establish a base line against which to measure improvement, and therapeutic effectiveness as a function of treatment type and time-in-therapy. The overall improvement rate across a heterogeneous collection of patients, therapists, and treatment modalities was 66 per cent, suggesting, conservatively, at least a moderately positive therapeutic effect in light of the judgment that “spontaneous” rates appear to be much lower in marital than in individual therapy. Evidence of deterioration in marital therapy also was discovered. No support was found for the contention that co-therapy is more effective than treatment of the couple by a single therapist. The needs of future research in the outcome of marital therapy are discussed and possible fruitful directions for such investigations suggested.