This chapter defines LSD's unique position among abused drugs; its history; its physiologic and psychologic effects; its therapeutic use; its marketing and distribution; and its relationship to the law. The drug has been used in the United States, first medically and then illegally, for more than 40 years. It was the subject of intense scientific interest and research until about 1980, when financial support for research dried up. Its nonmedical use began in the early 1960s. In the 1990s, hallucinogen research using human subjects is almost nonexistent, and the process of obtaining permission is extremely difficult. Pharmacologic research in animals has continued, however, and nonhallucinogenic derivatives of LSD have proven extremely useful in studies of serotonin receptors. LSD is available throughout the United States; overall supply appears to have been fairly stable since 1980, although regional fluctuations are common. The strength of the LSD available in the street market is variable but is almost certainly relatively pure. Notes