Although national surveys of drug use levels and patterns, public opinion, and health-related consequences of drug use have been conducted over the past two decades, difficulties occur when integrating survey findings because investigations of different population groups sometimes point to different conclusions about the prevalence of and trends in drug use. Two major population studies point to a downturn in drug use dating to the late 1970's. However, a study of drug-related deaths and hospital emergency room visits shows increases in these events in recent years. Studies also show that drug use, especially cocaine, continues to increase among criminals. Marijuana, the most widely used illicit drug, has decreased dramatically among the general population over the past decade, and all signs point to a downturn in use within the criminal subculture as well. There is still much to learn about drug use incidence and trends among the population as a whole and among groups most at risk for drug abuse. A continuing problem involves the validity of self-reported drug use. The author suggests that methodological differences in the studies being compared and lags between trends in the general population and certain subgroups account for most variations in drug use trend estimates. She concludes that overall illicit drug use is decreasing in the United States. Measures of drug use and the link between drug use and criminality are described, as well as the Drug Abuse Warning Network and the Drug Use Forecasting Project. 38 references, 8 notes, 6 tables, and 7 figures