This study evaluated the violence prevention effects of The Peacemakers Program, a school-based intervention for students in grades four through eight. The program includes a primary prevention component delivered by teachers and a remedial component implemented by school psychologists and counselors with referred students. The teacher-delivered component consists of a psychoeducational curriculum and procedures for infusing program content into the school environment. The study included almost 2,000 students in an urban public school system, with pre- and post-program assessment and comparison to a control group. There were significant, positive program effects on six of the seven variables assessed, including knowledge of psychosocial skills, self-reported aggression, and teacher-reported aggression, with a 41% decrease in aggression-related disciplinary incidents and a 67% reduction in suspensions for violent behavior. On some outcome variables, intervention effects were stronger for boys than girls and for middle school compared to upper elementary school students. © 2002 John Wiley & Sons, Inc.