Objective: The present study assesses program efficacy of Growing Against Gangs and Violence (GAGV), a primary prevention partnership with the U.K. Metropolitan Police Service (MPS), delivered in London schools with the aim of reducing gang involvement, delinquency, and violent offending and improving young people’s confidence in police. GAGV is partially derived from an American program, Gangs Resistance Education and Training (GREAT). Method: A qualitative process evaluation and randomized control trial (RCT) outcomes study were undertaken. Results: Findings indicate GAGV personnel were keen to enhance program fidelity and process implementation. The RCT did not demonstrate a statistically significant program effect. However, effect sizes (ESs) indicate the program was effective in reducing levels of gang membership and the frequency and variety of delinquency and violence in the short- and longer term. More robust evidence indicated GAGV also improved students’ attitudes toward police and reduced their adherence toward street code. Conclusions: The use of cohort- (not individual-) level data and missing data in the 1-year follow-up make it difficult to draw reliable and robust conclusions. However, results are encouraging. Several recommendations are suggested for GAGV, including curriculum design, regular evaluations, and expanding to include more schools. Limitations of this and similar evaluations also are discussed.