This monograph provides the 1975-2005 national trends in smoking, drinking, and illicit drug use among American secondary school students. It covers subgroup differences, attitudes and beliefs about use, social milieu, degree and duration of drug highs, initiation rates, prevalence and frequency, and trends in illicit drug and alcohol use. Drug use by age 45 is also covered. Most of the information reported deals with illicit use of controlled substances. The major exceptions are alcohol, cigarettes, smokeless tobacco, inhalants, nonprescription stimulants, creatine, and androstenedione. This report focuses attention on drug use at the higher frequency levels rather than simply to report proportions that have ever used various drugs. This is done to help differentiate levels of seriousness, or extent, of drug involvement. Over more than a decade--from the late 1970s to the early 1990s--the use of a number of illicit drugs declined appreciably among 12th-grade students, and declined even more among American college students and young adults. These substantial improvements--which seem largely explainable in terms of changes in attitudes about drug use, beliefs about the risks of drug use, and peer norms against drug use--have some extremely important policy implications. Another lesson that derives from the epidemiological data in this study is that social influences that tend to reduce the initiation of substance use also have the potential to deter the continuation of use by those who have already begun to use, particularly if they are not yet deeply involved in use. The following are appended: (1) Prevalence and Trend Estimates Adjusted for Absentees and Dropouts; (2) Definition of Background and Demographic Subgroups; (3) Estimation of Sampling Errors; (4) Trends by Subgroup: Supplemental Tables for Secondary School Students; and (5) Trends in Specific Subclasses of Hallucinogens, Amphetamines, Tranquilizers, and Narcotic Drugs Other Than Heroin. (Contains 187 tables and 96 figures.)