Of the many psychoactive plants used by indigenous peoples around the world, none has been the subject of such exhaustive botanical, chemical, and ethnographic study as the vine and drink known as ayahuasca or yagé. Ayahuasca—a Quechua word meaning “vine of the spirits”—is a psychotropic infusion with a millennia-old tradition considered sacred by millions of indigenous people throughout the Amazon rainforest basin. The magical-religious and biomedical interest in the properties and powers of ayahuasca led to the emergence of new cultural traditions related to the use of this drink in the 20th century. The book takes a multidisciplinary approach to analyzing traditional and modern uses of ayahuasca: indigenous, mestizo, religious and medical-therapeutic. The book touches on topics in the social and natural sciences, law, and the history of religions, providing a highly symbolic example of the processes of sociocultural encounter and synthesis that are taking place in this historical period, with repercussions that involve both scientific knowledge and legal-institutional reflection.