The psychedelic phenomenon, originated from the social and political movements of the early sixties of the last century in the United States, is now more than ever alive and extended, as also demonstrates the numerous websites, blogs and facebook groups dealing with it. Authors and interpreters of various kinds, natural and synthetic substances, related psychophysical techniques, archaeology, medicine and psychotherapy, institutions, figurative art, literature, films and music, groups and political and religious movements, symbolic places and events: these are the main areas of research developed here by Fulvio Gosso and Gilberto Camilla. With 490 entries, extensive bibliography and a large list of websites, this dictionary is the first complete job on the subject.