Available evidence suggests that a change in the social environment to one that is less tolerant of drug use is a likely mediator of program effects. There is inconsistent evidence that program effects differ in terms of such moderating variables as gender, ethnicity, grade, socioeconomic status, and urbanization. Results from the few available studies suggest that both high-risk groups and more typical groups benefit from prevention programs. Theories of drug use that emphasize social influences are strongly supported by available research. The results of prevention programs based on social learning are encouraging for the prevention of cigarette use onset, although evidence for alcohol use prevention is less clear. Because the analysis of mediation and moderation generates detailed information about how and for whom drug prevention programs work, mediating and moderating variables should receive increased attention as the field of school-based drug prevention research matures. 65 references.