This study examined the impact of treatment adherence and therapist competence on treatment outcome in a controlled trial of individual cognitive-behavioral therapy (CBT) and multidimensional family therapy (MDFT) for adolescent substance use and related behavior problems. Participants included 136 adolescents (62 CBT, 74 MDFT) assessed at intake, discharge, and 6-month follow-up. Observational ratings of adherence and competence were collected on early and later phases of treatment (192 CBT sessions, 245 MDFT sessions) by using a contextual measure of treatment fidelity. Adherence and competence effects were tested after controlling for therapeutic alliance. In CBT only, stronger adherence predicted greater declines in drug use (linear effect). In CBT and MDFT, (a) stronger adherence predicted greater reductions in externalizing behaviors (linear effect) and (b) intermediate levels of adherence predicted the largest declines in internalizing behaviors, with high and low adherence predicting smaller improvements (curvilinear effect). Therapist competence did not predict outcome and did not moderate adherence-outcome relations; however, competence findings are tentative due to relatively low interrater reliability for the competence ratings. Clinical and research implications for attending to both linear and curvilinear adherence effects in manualized treatments for behavior disorders are discussed. (Contains 2 tables, 1 figure, and 2 footnotes.)