Hallucinogens that are likely to be abused include the ergot hallucinogen lysergic acid diethylamide (LSD), which is the prototype of these drugs of abuse; other indolealkylamines such as psilocybin ("magic mushrooms") and dimethyltryptamine (DMT; also contained in the South American beverage ayahuasca); and the phenalkylamines, including mescaline and dimethoxymethylamphetamine (DOM; "STP"). The hallucinogens are a group of drugs that produce thought, mood, and perceptual changes. Depending on dose, expectation (set), and environment (setting), they also can induce euphoria and a state similar to a transcendental experience. Many drugs can cause auditory and/or visual hallucinations. These hallucinations can be present as part of a delirium, when accompanied by disturbances in judgment, orientation, intellect, memory, and emotion. Delirium also can result from drug withdrawal (e.g., associated with sedative-hypnotic and/or alcohol withdrawal). When used in the context of substance abuse, the term hallucinogens generally refers to a group of compounds that alter consciousness without producing delirium, sedation, excessive stimulation, or impairment of intellect or memory. Acute abstinence from repeated use of hallucinogens does not produce a withdrawal syndrome, and pharmacotherapy rarely is required for treatment of toxic effects. We therefore focus on the acute and chronic effects of these drugs and the psychosocial interventions necessary to treat their associated adverse reactions. LSD is discussed as the prototypical hallucinogen.