Despite the widespread use of psychotherapy for patients with substance use disorders, the effectiveness of psychotherapists conducting such treatment has received little research attention. In this paper, empirical studies of therapists' differences in patient outcome and dropout rates are comprehensively reviewed. The main conclusions are that therapists show diverse rates of effectiveness, and that such differences appear independent of both therapists' professional background and of patient factors at the start of therapy. The primary therapist characteristic thus far associated with higher effectiveness is the possession of strong interpersonal skills. Guidelines for research on therapist effectiveness are presented.