This report describes one of the largest and longest running school-based violence prevention programs in the country, the Resolving Conflict Creatively Program (RCCP), and discusses the results of a rigorous evaluation of the program's effectiveness conducted by the National Center for Children in Poverty (NCCP) at Columbia University's Joseph L. Mailman School of Public Health. The report is designed to inform policymakers, program developers and managers at the local level, and other opinion leaders and decision makers of effective strategy for directly addressing the problem of violence among children and youth. Founded in 1985, the RCCP now serves approximately 9,000 students in 60 New York City public schools. The program is designed to promote constructive conflict resolution and positive inter-group relations. It is based on the philosophy that aggressive and violent behavior is learned and therefore can be reduced through education. The program is built around a set of core skills: communicating clearly and listening carefully, expressing feelings and dealing with anger, resolving conflicts, fostering cooperation, appreciating diversity, and countering bias. These skills are learned through a curriculum taught by teachers receiving both initial training and ongoing follow-up and support from RCCP staff developers. The RCCP is also implemented through the training of student-based peer mediation groups and school administrators, and by continued outreach to parents. Overall, NCCP's evaluation found that the RCCP had a significant positive impact when teachers taught a high number of lessons from the RCCP curriculum. Among other findings, children receiving a high number of lessons had significantly slower growth in self-reported hostile attributions, aggressive fantasies, and aggressive problem-solving strategies, as well as in teacher-reported aggressive behavior, compared to children receiving a low number of lessons or no lessons at all. (Appendices include related evaluations of the RCCP implementation, NCCP design for the evaluation of the RCCP, and implications of the RCCP evaluation for evaluation research.) (EV)