In this naturalistic study, 262 audiotaped psychotherapy sessions-randomly drawn from 81 individual therapies from eight different psychotherapy approaches-were rated completely on treatment adherence using a newly developed rating manual. In the therapy sessions, a relatively low percentage of treatment specific interventions (ranging from 4.2% to 27.8%) was found for all eight approaches, 50% to 73% of the interventions were nonspecific or common, and approximately 18% to 27% were intervention techniques from other approaches. Different types of psychotherapy differed highly significantly in levels of treatment adherence. There was no statistically significant association between the type of psychotherapy and its outcome, or between the degree of therapists' treatment fidelity and the treatment outcome. However, there were significant associations between therapists' degree of professional experience, clients' initial psychological burden, and treatment response. Clients' severity of psychological problems prior to treatment predicted quality of therapeutic alliance while therapists' treatment adherence was predicted by therapists' professional experience and by the quality of the therapeutic alliance. We discuss the seemingly indirect importance of treatment adherence for psychotherapy outcome that we found in this study in relation to findings from other studies and in the context of the role of schools within psychotherapy.