The abuse of alcohol and other drugs is the greatest cause of disability and perhaps death in adolescents and young adults. Over half of all accidents in the 15-24 year age group are due to motor vehicle fatalities, of which 45 percent involve alcohol abuse. Substance abuse prevention strategies are most likely to be effective when based on an understanding of the causes of drug abuse. Because there is no one cause of drug experimentation, drug abuse, and progression to drug dependence, no single prevention method will be completely effective. An understanding of factors that lead to host susceptibility and resistance is essential to prevention planning. In addition to individual variations in biological response to drugs, prevention planners must consider the biological effects of psychoactive drugs on all users. Attitudes and messages of teenagers' environment strongly influence their behavior, and parental patterns of drug use and attitudes about the acceptability of drug use also have a strong effect. Theories of drug abuse causation are examined including problem behavior, social learning, stage, and biopsychosocial models. Frameworks for the study and prevention of substance abuse are examined that focus on the concept of high-risk adolescents and social stresses. An overview of prevention programs is presented, the early identification of high-risk youth is addressed, and the role of primary care physicians in drug and alcohol prevention is discussed. 34 references, 4 tables, and 4 figures