This review was undertaken in recognition of the mounting public health and social problems associated with adolescent sexual behavior and the importance of basing school-affiliated programs designed to reduce sexual risk-taking behavior on sound research. The authors were commissioned by the Division of Adolescent and School Health within the Centers for Disease Control and Prevention, Public Health Service, to review carefully the research on these programs and to assess their impact on behavior. The authors identified 23 studies of school-based programs that were published in professional journals and measured program impact on behavior. They then summarized the results of those studies, identifying the distinguishing characteristics of effective programs, and citing important research questions to be addressed in the future. Not all sex and AIDS education programs had significant effects on adolescent sexual risk-taking behavior, but specific programs did delay the initiation of intercourse, reduce the frequency of intercourse, reduce the number of sexual partners, or increase the use of condoms or other contraceptives. These effective programs have the potential to reduce exposure to unintended pregnancy and sexually transmitted disease, including HIV infection. These programs should be replicated widely in U.S. schools. Additional research is needed to improve the effectiveness of programs and to clarify the most important characteristics of effective programs.