This survey has been the primary source of estimates of the prevalence and incidence of illicit drug, alcohol, and tobacco use in the U.S. population since 1971. The survey is based on a nationally representative sample of the civilian, noninstitutionalized population of the United States age 12 years old and older. A sample of 25,500 persons was interviewed for the 1998 survey; augmented samples were included for California and Arizona. Findings on illicit drug use show that an estimated 13.6 million Americans were current users of illicit drugs in 1998, meaning they used an illicit drug at least once during the 30 days prior to the interview. Although this number is slightly less than the 13.9 million estimate for 1997, the difference is not statistically significant. By comparison, the number of current illicit drug users was at its highest level in 1979, when the estimate was 25.0 million. For youths aged 12-17, 9.9 percent reported current use of illicit drugs in 1998. This is a statistically significant decrease from the estimate of 11.4 percent in 1997. Of youths age 12-17, 8.3 percent were current users of marijuana in 1998. The prevalence of marijuana use among youth did not change significantly between 1997 (when it was 9.4 percent) and 1998 (when it was 8.3 percent). An estimated 1.8 million Americans age 12 and older were current users of cocaine in 1998, a slight but statistically insignificant increase from 1997. Detailed data are also provided on alcohol use, tobacco use, perceived risk and availability of drugs, and trends in new use of substances. One section of the report focuses on drug use in California and Arizona. Appended methodological discussions, 69 references, and 61 tables