The world-wide chronology, biological diversity, and geographical range of human use of psychoactive drug plants are neither complete nor unchangeable. Numerous psychoactive plants and regions are under-researched and require initial or additional research on this topic. Many scholars would believe it to be safe to assume that most psychoactive plants have a longer history of use than that recorded here. In any case, it is hoped that this paper provides a starting point from which other scholars may build upon. With a critical application of archaeology, paleobotany, anthropology, cultural geography, history, and other research disciplines the picture may become more complete. The evidence provided here should reveal the potential for such work in the future.