Central to the debate over the implementation of empirically supported treatments is whether therapist skill has a measurable, positive relationship with client outcome. The fidelity and skill with which therapists deliver treatments have been studied under the constructs of adherence and competence. Evidence for a relationship between adherence and competence and client outcomes has been mixed, possibly due to small sample sizes, potentially inadequate measures for rating therapists’ skill, and limited statistical methods. The current study used a data set in which 91 therapists provided services to 384 clients from the Assertive Adolescent and Family Treatment project. Therapists trained to deliver the Adolescent Community Reinforcement Approach (A-CRA) submitted audiorecorded sessions to independent raters during their training process. Measures of adherence and competence derived from session ratings were examined in a multilevel model for associations with client substance use outcomes at 3-, 6-, and 12-month follow-ups. Therapist competence was significantly predictive of decreases in clients’ days of substance use. Therapist adherence was not predictive of client substance use outcomes in the full sample; however, when only those clients who completed the 12-month follow-up were included, between-therapists adherence was found to be predictive of a decrease in client substance use. This study adds to the evidence that competence in the delivery of treatment is associated with better client treatment outcomes. Adherence may be associated with better treatment outcomes as it was in follow-up completers. Future research should investigate the nature of the relationship between protocol-specific adherence and competence and client outcomes.