An overview of drug use patterns notes that experimentation with tobacco, alcohol, or other drugs has become common among youth and that experimentation too often results in regular patterns of use that are characterized by both psychological and physical dependence. However, alcohol use and cigarette smoking generally have declined, as has the use of sedatives, tranquilizers, opiates, and hallucinogens. Data indicating that different types of problem behaviors are highly associated suggests that they may share common causes. The data suggest the need for comprehensive and early prevention efforts. Earlier detection may help drug abusers before severe problems. However, until better assessment procedures are developed and treatment programs are designed specifically for adolescents, youth and their families will continue to invest large amounts of time, effort, and expense in problems that prove to be without long-term value. Despite methodological advances, validity is a crucial issue in evaluations. Program designer must also address theoretical and practical issues when disseminating an intervention developed in a research setting. Figures, tables, author and subject indexes, and approximately 200 references