Objective: Violence is a leading cause of morbidity and mortality among young people. Primary preventive programs aimed at reducing the involvement of young people in violence are often implemented in a school setting. This systematic review evaluated the effectiveness of universal school-based programs aimed at the primary prevention of violence in 11- to 18-year-olds. Method: A predefined search strategy was used to search various sources (i.e., databases, gray literature, previous reviews, and reference lists of included studies) for randomized design trials and quasi-experimental design trials published between 2002 and March 2014. After screening 8,051 abstracts, 21 studies that satisfied the inclusion and exclusion criteria were identified. These studies evaluated 16 different programs based mainly in the United States. Results: Due to the heterogeneity, a meta-analysis was not possible; thus, a narrative synthesis was reported. The most effective interventions utilized social development and social norms components. Attitudes toward violence was the most frequently measured outcome, with 6 studies reporting a beneficial effect and 2 reporting no effect; 3 of the 6 studies examining violent behavior demonstrated a small beneficial effect; and 4 of the 7 studies examining physical aggression demonstrated a small beneficial effect. Conclusions: In general, this review found limited evidence of the effectiveness of universal school-based programs in the primary prevention of violence in 11- to 18-year-olds; however, those that combined social development and social norms approaches appeared to be the most effective. Additional qualitative research and process evaluation is required to establish the processes that underpin the success or failure of such programs in order to inform their refinement and the future development of effective programs. (PsycINFO Database Record (c) 2016 APA, all rights reserved)