Many people assume that experiments with hallucinogens began with Timothy Leary and the psychedelic revolution of the fifties and sixties. In fact, as this study demonstrates, psychedelics have been used by human societies in every part of the world for ritual and spiritual purposes for millennia. The author reveals evidence - from linguistics, ethnobotany, biology and other fields - for the psychedelic experiences of various prehistoric cultures. He also argues that our modern culture is eccentric in its refusal to integrate the profound experiences offered by these natural substances into our own spiritual life and traditions.