Psychoactive plants have played an important role in medicine, religion, ritual life, and recreation since ancient times. In shamanic religions, which appear to have dominated throughout much of human pre-history, trance induced by psychoactive plants and other techniques permits direct contact with the divine. For this reason, plant hallucinogens and other psychoactive botanicals have been considered by cultures throughout history as "plants of the gods": sacred substances that bring knowledge, power, healing, and mystical insight, but that must be used with utmost respect and caution. With the spread of Christianity, and especially since the Inquisition and Conquest of the New World, the religious use of psychoactive plants has been severely and sometimes violently suppressed. Western scientific and popular interest in psychoactive plants enjoyed a resurgence in the mid-twentieth century, though the excesses of the 1960s "psychedelic era" provoked a backlash, exacerbating existing biases within the scientific, medical, and law-enforcement establishments. Psychoactive compounds produce their peculiar effects on consciousness by mimicking the chemical structures of neurotransmitters or otherwise altering the transmission of nerve impulses. Over the past two hundred years, chemical and physiological studies of natural psychoactive compounds and their synthetic derivatives have resulted in major contributions to medicine and neuroscience. This chapter presents an overview of twenty-two important psychoactive plants used in religious or ritual settings throughout the world, with supplementary information on ten additional species. The cultural and historical background for each plant is presented alongside pertinent botanical, chemical, and pharmacological information. An appendix provides a summary of the names, traditional and biomedical uses, and active components of plants discussed in the text. A general introduction and concluding discussion help set the topic of psychoactive plant use within the intertwined historical, social, philosophical, scientific, and contemporary legal contexts.