Recently, there has been a resurgence of interest in the study of classic serotonergic hallucinogens, now widely referred to as classic psychedelics. These studies include fundamental molecular and cellular neuroscience and pharmacology, neuroimaging, and psychological experiments. In the early days of classic psychedelic research during the 1950s through the 1970s, classic psychedelics were examined for their therapeutic potential to treat disorders ranging from addiction to schizophrenia. After scheduling laws essentially halted classic psychedelic research worldwide for decades, beginning in the mid-2000s, a few select research groups were given regulatory approval to reinitiate clinical investigations with psilocybin. These studies demonstrated safety and allowed for the development of standardized methodology for conducting clinical trials with psilocybin and other classic psychedelics. There are now data from several clinical trials approved by the proper regulatory agencies in the United States, Europe, and elsewhere, suggesting safety and potentially profound efficacy to treat a variety of psychiatric disorders including anxiety, depression, and addiction.