Objective: Therapist adherence has been shown to predict clinical outcomes in family therapy. In prior studies, adherence has been represented broadly by core principles and a consistent family (vs. individual) focus. To date, these studies have not captured the range of clinical skills that are represented in complex family-based approaches or examined how variations in these skills predict different clinically relevant outcomes over the course of treatment. In this study, the authors examined the reliability and validity of an observational adherence measure and the relationship between adherence and outcome in a sample of drug-using adolescents who received brief strategic family therapy within a multisite effectiveness study. Method: Participants were 480 adolescents (age 12-17) and their family members, who were randomized to the Brief Strategic Family Therapist treatment condition (J. Szapocznik, U. Hervis, & S. Schwartz, 2003) or treatment as usual. The adolescents were mostly male (377 vs. 103 female) and Hispanic (213), whereas 148 were White, and 110 were Black. Therapists were also randomly assigned to treatment condition within agencies. Results: Results supported the proposed factor structure of the adherence measure, providing evidence that it is possible to capture and discriminate between distinct dimensions of family therapy. Analyses demonstrated that the mean levels of the factors varied over time in theoretically and clinically relevant ways and that therapist adherence was associated with engagement and retention in treatment, improvements in family functioning, and reductions in adolescent drug use. Conclusions: Clinical implications and future research directions are discussed, including the relevance of these findings on training therapists and studies focusing on mechanisms of action in family therapy. (Contains 2 tables and 1 footnote.)