This chapter discusses in detail the way that epidemiological research has and can influence the formulation of prevention programs and the research designed to evaluate them. There are eight basic ways in which such studies can inform prevention efforts by providing a dynamic assessment of (1) drug use or drug-related problems that need to be prevented, (2) the age of onset of drug use, (3) subgroups in the population most at risk in terms of demographic and lifestyle characteristics, (4) the changing backdrop against which the effects of prevention programs should be evaluated, (5) the importance of key intervening variables including attitudes and beliefs, (6) behavioral and moral norms with regard to drug use among adolescents and their peer groups, (7) the extent to which major classes of prevention programming are reaching targeted populations, and (8) the combined effectiveness of all forces in the society that tend to reduce drug abuse. 6 figures, 6 tables, 2 notes, and 23 references