Hallucinogenic agents represent an old and very large class of drugs. Nearly every major civilization throughout history has had a preferred drug of abuse or mind-altering substance. In some instances these have been hallucinogens or hallucinogen-related agents or plant products. Various agents can produce hallucinogenic episodes, and terms used to describe such agents include hallucinogens, psychotomimetics, psychedelics, inebriants,and intoxicants. Many agents can be found in this general class of psychoactive agents. More recently, certain hallucinogens have been included in the loose collection of agents termed club drugs, party drugs, or rave drugs. It is clear, however, that membership in these latter categories is not pharmacologically based and that most agents bearing this appellation are not hallucinogens. Do agents as structurally diverse as (+)lysergic acid diethylamide ([+]LSD), phencyclidine (PCP, angel dust), tetrahydrocannabinol (THC; a constituent of marijuana), amphetamine, and mescaline all produce the same (or a common) effect? Do they all work via a common pharmacological mechanism? Studies conducted over the past several decades indicate they do not (see Glennon 2002 for a review).