Psychedelics are psychoactive substances that powerfully alter perception, mood, and cognitive processes. Despite the published adverse reactions, they are now considered physiologically safe and do not produce dependence or addiction. Known and used for millennia, their human use predates written history, and they were employed by early cultures in a ­variety of sociocultural and ritual contexts. Although there has been a western interest for over half a century in the therapeutic use of classic hallucinogens to treat substance use disorders and other selected psychiatric disorders, medical research with these drugs was halted in the early 1970s, leaving many questions unanswered.