Used meta-analysis to review 177 primary prevention programs designed to prevent behavioral and social problems in children and adolescents. Findings provide empirical support for further research and practice in primary prevention. Most categories of programs produced outcomes similar to or higher in magnitude than those obtained by many other established preventive and treatment interventions in the social sciences and medicine. Programs modifying the school environment, individually focused mental health promotion efforts, and attempts to help children negotiate stressful transitions yield significant mean effects ranging from 0.24 to 0.93. In practical terms, the average participant in a primary prevention program surpasses the performance of between 59% to 82% of those in a control group, and outcomes reflect an 8% to 46% difference in success rates favoring prevention groups. Most categories of programs had the dual benefit of significantly reducing problems and significantly increasing competencies. Priorities for future research include clearer specification of intervention procedures and program goals, assessment of program implementation, more follow-up studies, and determining how characteristics of the intervention and participants relate to different outcomes.