Ethnopharmacology is a relatively new science that studies the cultural aspects of substances (plants, animals, minerals) and their biological characteristics and activities. While investigating and identifying compounds and the various uses they have in indigenous and non-indigenous groups, this science is also involved in studying the bio-activity of these materials. Ethnopharmacology is connected with other ethno-sciences, especially ethnobotany, which studies the uses of plants by human groups and their chemical and biological aspects. Plant development is part of nature but also part of the human culture, since man has been selecting plant materials from millennia for their qualities and characteristics. Humans use plants to eat, to hunt, to build houses, for killing, and from the rich variety of uses man had been given to plants, some kinds of uses involves the magical. Ayahuasca, a psychoactive Amazonian plant preparation, had been and still is used within the context of the sacred, the religious, the divinatory and the therapeutic. Ayahuasca is an excellent bio-active material to be investigated from an ethnopharmacological point of view, since its various uses, in the past and in the present, by many different human groups and for a high diversity of objectives, warrant an interdisciplinary approach. Although many human groups have been using different kinds of flora for reaching and connecting with the sacred, the hallucinogenic botany seems to occupy a very special place in some cultures. The high relevance of these substances is evident in many human groups. In the present book, several authors present to us some of the uses that ayahuasca had and still has in indigenous, mestizo and urban groups. Also, other chapters illustrate the various uses that ayahuasca have in modern science. This pan-Amazonian brew is used actually not only in Brazil, Colombia, Peru or Ecuador, where indigenous groups have been using this plant preparation for centuries, but in North America, Europe and Asia, where non-indigenous groups have been using this brew for many years now. It is an attempt of the present work to try to show the rich diversity that exists around ayahuasca. Because ethnopharmacology involves social aspects of bio-active compounds, the two first articles of the book are made considering the anthropological, cultural and social aspects of ayahuasca. In the first article, Luis Eduardo Luna shows how ayahuasca is used by indigenous and mestizo populations and in the second article Sandra Lucia Goulart discusses the context of the Brazilian ayahuasca religions. Luna and Goulart illustrate the diversity of concepts and ideas that these groups have regarding ayahuasca, discuss concepts and practices of healing and disease in these groups, and compare the ways in which ayahuasca is experienced and explained in these groups. The next articles are about pharmacological aspects of ayahuasca, to complement the social and cultural aspects. On the third article, José Carlos Bouso and Jordi Riba describe the pharmacological and neuropsychiatric aspects of this Amazonian brew. Their work is based in many years of clinical trials with acute ayahuasca administration to healthy volunteers and also on the investigations regarding the possible consequences of the long term consumption of this centenary potion. On the fourth article, Paulo César Ribeiro Barbosa illustrates a case report of a person that has taken ayahuasca for the first time. The possible therapeutic effects of ayahuasca are investigated in the context of the acute and post-acute effects of the brew. On the fifth article, Flávia de Lima Osório and collaborators explore the therapeutic potentials of one of the main ayahuasca alkaloids – harmine – for the treatment of depression, and they also discuss the curative potentials of the brew itself in this psychological disequilibrium. Still on the pharmacological perspective, but also involving psychiatry, the last two articles talk about the possible interactions of ayahuasca and cannabis in humans. One of the articles, which was wrote by me, is a theoretical overview of the eventual interactions – positive, neutral, or negative – that can potentially happen when humans take ayahuasca and cannabis together. The last article, wrote by me and Rick J. Strassman, shows a case report of a young man that had psychotic episodes related directly to the consumption of ayahuasca and cannabis. It is the main intention of the present book to make accurate and easy information regarding as many as possible aspects of ayahuasca accessible to the public. It is hoped that this little book will bring light to an issue that, in many cases, is still debated in obscurity.