As a nation, the U.S. invests heavily in community-based organizations to conduct interventions, proven through research, to reduce the high rates of alcohol and drug use among teens. Much less is invested in helping communities implement these programs with quality. Although many evidence-based programs exist to address alcohol and drug use, communities face difficulty implementing them and achieving the same outcomes as researchers. This "gap" is because resources are limited, prevention is complex, and communities often lack the capacity—or the knowledge, attitudes, and skills—needed to implement "off the shelf" programs well. Common ways to bridge this gap, such as information dissemination, fail to change practice or outcomes at the local level in part because it does not sufficiently address capacity of community practitioners. Therefore, building a community's capacity is a method that could improve the quality of implementation and outcomes. Getting To Outcomes (GTO) is an implementation support intervention shown to improve the capacity of practitioners to conduct evidence-based programs, but its impact on fidelity and youth outcomes has not been studied. The study—Preparing to Run Effective Prevention (PREP)—will be a cluster randomized controlled trial using primary data from middle school youth (990) and program staff from 33 cooperating Boys and Girls Clubs sites (Sites) to assess how GTO impacts the fidelity, outcomes, and costs of an evidence-based program to prevent alcohol and drug use (Project CHOICE, PC). All 33 sites will receive training in PC. 16 Sites will conduct PC in a way typical of community-based organizations (PC only); 17 Sites will conduct PC augmented by GTO, comprised of training, onsite technical assistance, and a written GTO manual. The study will: (1) Assess the use of GTO and compare Sites' capacity between PC+GTO and PC only; (2) Compare PC fidelity between PC+GTO and PC only groups; (3) Compare alcohol and marijuana outcomes of middle-school youth in the PC+GTO (n=510) and PC only (n=480) groups; and (4) Assess the cost-effectiveness of GTO' impact. To address these aims we will collect data on the delivery and utilization of GTO (e.g., method of delivery, duration, topics); staff capacity to implement research-based interventions; observations of program delivery (fidelity monitoring); youth participants' use of, and attitudes towards, alcohol and marijuana via a survey at baseline and 3 and 6 months follow-up; and costs of both PC alone and PC+GTO. Analyses will examine differences between intervention (PC+GTO) and control (PC only) Sites' capacity, PC fidelity, outcomes, and costs over time, accounting for clustering within site. The PREP study could expand implementation research and serve as a model for how the government and other agencies support strong implementation of evidence-based programs in a wide array of domains. These outcomes are important to providing opportunities for youth to become healthy and productive adults.