The major data sources for this chapter include: (1) the National Household Survey on Drug Abuse; (2) Monitoring the Future: A Continuing Study of the Lifestyles and Values of Youth; (3) the Drug Abuse Warning Network; (4) the Drug Use Forecasting Project; and (5) the Community Epidemiology Work Group. The level of LSD use varied little between the late 1970s and early 1990s and significant increases in LSD supplies have not been reported at the national level in recent years. LSD users are usually white, male and middle to upper class and have probably used alcohol, marijuana and possibly inhalants and over-the-counter drugs. The average user tries LSD at age 17. Even among the small number of high school students who continue use beyond one or two experiments, most will have stopped by the time they reach their early 20s. LSD use only rarely results in a reaction requiring emergency medical care. It is very likely that an adolescent using LSD will have already tried and perhaps be continuing to use other drugs as well. Among younger students, inhalation of potentially lethal substances such as solvents and butane is more common than LSD use. Tables, figures