Universal school-based programs to reduce or prevent violent behavior are delivered to all children in classrooms in a grade or in a school. Similarly, programs targeted to schools in high-risk areas (defined by low socioeconomic status or high crime rates) are delivered to all children in a grade or school in those high-risk areas. During 2004-2006, the Task Force on Community Preventive Services (Task Force) conducted a systematic review of published scientific evidence concerning the effectiveness of these programs. The results of this review provide strong evidence that universal school-based programs decrease rates of violence and aggressive behavior among school-aged children. Program effects were demonstrated at all grade levels. An independent meta-analysis of school-based programs confirmed and supplemented these findings. On the basis of strong evidence of effectiveness, the Task Force recommends the use of universal school-based programs to prevent or reduce violent behavior.