The use of licit and illicit drugs by American young people has been a source of major policy and public health concerns for the United States since the mid-1960s. The use of these substances is a leading cause of eventual disease and death in the population, but it also contributes in important ways to mortality and morbidity during adolescence. Monitoring the Future, which is now in its 29th year, has become one of the nation's most relied-upon sources of information on changes taking place in licit and illicit psychoactive drug use among American adolescents, college students, and young adults. The study has tracked and reported the use of an ever-growing array of such substances in these populations. This annual series of monographs, written by the study's investigators and published by the National Institute on Drug Abuse is one of the major vehicles by which the epidemiological findings from the study are reported. The present two-volume monograph reports findings through 2003. (A companion series of annual reports provides a much briefer, advanced synopsis of the key findings from the latest surveys of secondary school students.) Appended are: (1) Prevalence and Trend Estimates Adjusted for Absentees and Dropouts; (2) Definition of Background and Demographic Sub Groups; (3) Estimation of Sampling Errors; (4) Supplemental Tables for Secondary School Students: Trends by Subgroup; and (5) Trends in Specific Subclasses of Hallucinogens, Amphetamines, Tranquilizers, and Narcotics Other Than Heroin. An index of drugs is also included. (Contains 146 tables and 95 figures.)