This study evaluated the violence prevention effects of The Peacemakers Program, a schoolbased 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.During the past decade, major efforts have been made to prevent youth violence by means of school-based interventions (Powell, Muir-McClain, & Halasyamani, 1995;Price & Everett, 1997). A number of primary prevention programs have been developed, and several have become widely implemented. However, implementation has proceeded faster and more extensively than has accumulation of empirical support for the effectiveness of these interventions (Butler, Adams, Tsunokai, & Neiman, 1998;Flannery & Williams, 1999;Howard, Flora, & Griffin, 1999; Kellerman, FuquaWhitley, Rivara, & Mercy, 1998;Larson, 1994;1998). While some interventions for young children have demonstrated effectiveness, universal prevention programs for adolescents have generally not been able to produce positive evaluation results (Farrel & Meyer, 1997, Orpinas et al., 2000. The purpose of the present study was to evaluate the effects of a preventive intervention for upper elementary and middle school students called The Peacemakers Program (Shapiro, in press-a, in press-b).The above reviews identified only a few school-based, universal violence prevention programs with published studies reporting clear, positive intervention effects. There is more evidence supporting targeted interventions than universal programs, and there are many more general prevention and psychosocial skill-building programs than interventions focusing specifically on violence, but the below review examines only our area of interest, namely, school-based, universal, violence prevention programs.The Second Step program (Committee for Children, 1992) is a curriculum-based, teacherdelivered intervention that teaches psychosocial skills including anger management, empathy, impulse control, and problem solving. In a study with 2nd-and 3rd-grade students, Grossman et al. (1997) found that this intervention resulted in decreased aggression as measured by behavioral observers, although parent-repo...