Following an overview of key findings for the study as a whole, study design and procedures are described. A chapter on the prevalence of drug use among high school seniors addresses the prevalence and frequency of drug use in 1990, noncontinuation rates, and prevalence comparisons for important subgroups. Trends in drug use among high school seniors encompass trends in prevalence for 1975-90, trends in noncontinuation rates, and comparisons among subgroups in trends in prevalence. A chapter on drug use at an earlier grade level considers the incidence of use by grade level and trends in use at earlier grade levels. Data on the degree and duration of drug highs are followed by data on the attitudes and beliefs about drugs among seniors. Issues addressed in the latter data are the perceived harmfulness of drugs, personal disapproval of drug use, attitudes regarding the legality of drug use, and the legal status of marijuana. Information on the social milieu for seniors includes the perceived attitudes of parents and friends, exposure to drug use by friends and others, implications for the validity of self- reported usage questions, and the perceived availability of drugs. Other topics for which data are provided are the use of nonprescription stimulants, the use of marijuana on a daily basis, racial/ethnic differences in drug use, an explanation of the recent declines in marijuana use and cocaine use, changes in drug use during pregnancy, a theory of drug epidemics, and other data on correlates and trends. 32 figures and appended prevalence and trend estimates adjusted for absentees and dropouts