The purpose of the Monitoring the Future study is to serve a social monitoring or social indicator function intended to characterize accurately the levels and trends in certain behaviors, attitudes, beliefs, and conditions in the population. Other purposes include helping to determine what types of young people are at greatest risk for developing various patterns of drug abuse; and gaining a better understanding of the lifestyles and value orientations associated with various patterns of drug abuse. Findings show that over more than a decade (late 1970's to early 1990's) there were very appreciable declines in use of a number of illicit drugs among 12th grade students and even larger declines in their use among American college students and young adults. These improvements 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. One policy implication is that the various substance-using behaviors among young people can be changed. Another is that demand-side factors appear to have been pivotal in bringing about those changes. The reported levels of availability of marijuana, as reported by high school seniors, held fairly steady throughout the life of the study. The perceived availability of cocaine actually was rising during the beginning of the sharp decline in cocaine and crack use, which occurred when the risks associated with that drug suddenly rose sharply. This study has demonstrated that changes in perceived risk and disapproval have been important causes of change in the use of several drugs. 72 figures, 30 tables