This dissertation contributes to the scant and inconclusive literature on fidelity and alliance effects on outcomes of manualized youth behavioral health intervention. The aims of this study are to test the comparative effects of fidelity and alliance on (1) treatment progress, (2) change in behavioral health symptoms, and (3) recidivism among juvenile offenders receiving Functional Family Therapy (FFT) – a community-based family therapy intervention. The concurrent effects of fidelity and alliance on FFT outcomes have not been previously tested, which can help to better understand the relative importance of the technical and relational aspects of delivering an intervention effectively. This study involved secondary analysis of data collected for a statewide evaluation of FFT for delinquent youth in Maryland. Using a single-group longitudinal design, treatment progress and change in behavioral symptoms were observed at discharge, and recidivism was observed at 12 months post-discharge. Mixed-effects models regressed treatment progress, behavioral symptoms change, and recidivism on fidelity, alliance, and other covariates. Therapist random intercepts estimated between-therapist variance in each outcome when controlling for fidelity and alliance individually, and both simultaneously, to better understand the role of each in differentiating the effectiveness of FFT across different practitioners. Stronger alliance was associated with greater reductions in youth behavioral symptoms and recidivism for any offense. Higher fidelity was associated with further progress through treatment and reduced recidivism for adjudicated offenses. Neither fidelity nor alliance substantively explained between-therapist variance in outcomes. Results suggest that the relative importance of fidelity and alliance is contingent upon the goals of implementing the intervention, with alliance being more salient for clinical outcomes and fidelity for program retention and delinquency status. For public systems-involved youth with emotional and behavioral challenges, both clinical functioning and systems involvement are critical outcomes, underscoring that both fidelity and alliance are essential for effectively serving this population.