In 1957, R. Gordon Wasson, a professional banker and amateur mycologist, inadvertently launched a profound cultural change that has come to be called the Psychedelic Revolution, by publishing an account of his experience with a Mazatec shaman in Hautla de Jiménez in the mountains of central Mexico. The article appeared in Life magazine and was intended as publicity for his forthcoming Russia, Mushrooms, and History, in which he and his Russian-born wife Valentina Pavlovna pursued their lifelong fascination with their dichotomous attitudes toward fungi, which had led them to suspect a cultural taboo upon a sacred object. In 1968 he traced this taboo back to the Vedic Soma, which he identified as a psychoactive mushroom. The identification, if correct, implied that there should be evidence for a similar sacred role for the mushroom in other regions in antiquity where the migrating Indo-European people settled. In 1978, he proposed such a role for the visionary potion that was central to the mystical experience of the Greek Eleusinian Mystery, that was celebrated annually for two millennia at a sanctuary near Athens. The possibility that the ancient Greeks indulged in chemically altered consciousness is antithetical to Europe’s idealization of Classical antiquity and the proposal was largely ignored. Mushrooms, however, were fundamental to social norms and religious observances in the celebration of Dionysus, and figured in other Mystery cults and in the foundational traditions of many cities, including Mycenae and Rome. The Soma sacrament as the Persian haoma was proselytized to the West by the Zoroastrian priests of Mithras and became a major cohesive indoctrination for the Emperors, army, and bureaucrats who administered the Roman Empire. It survived the Conversion to Christianity in the knighthoods of late antiquity and the medieval world, and was assimilated to the Eucharist of certain of the ecclesiastical elite. Key Words: mushroom, ergot, Dionysus, Apollo, Mithraism, Renaissance art, prehistoric rock art