In social sciences, theories are abstractions that aim to represent part of the world by a simpler structure. Theories simplify decisionmaking; predict the future; and allow generalizations from one instance, location, or group to another. Among the accepted theories used to analyze drug use onset are predictors of drug use; knowledge, attitude, and behavior (KAB) change; social learning theory; social environment; social bonding; and intrapsychic variables. These authors delineate 12 functions or roles of theory in the development and testing of drug use prevention programs. In terms of program development, theories can link theoretical elements and program components, reach consensus regarding the magnitude of program effects, and suggest the need for comprehensive programs. In prevention program evaluation, theories can link program components and intervening variables, inform program implementation and dissemination, inform external validity, target audience characteristics, inform social environment characteristics, inform construct validity, clarify cause-effect relationships and intervening variables, suggest short-term versus long-term effects, suggest unintended effects, inform measurement, and help explain effects of nontheoretically derived programs. More generally, theories of behavior allow researchers to discriminate between program and theory failure, contribute to social science knowledge, and contribute to research efficiency. 1 table, 4 figures, 4 notes, and 83 references