The therapeutic, initiative or religious use of hallucinogens is a constant in the history of civilizations. This subject is currently being studied in the United States, Europe and other countries after a long period of stagnation. Only with the advent of scientific medicine and the movements of temperance and prohibition of alcohol in the 19th century was the speech on psychoactive substances radically changed in a reducing sense and social control excluding any anthropological and symbolic understanding of their use. This article is published in two parts. This first part specifies the definitions of the terms "hallucinogens, psychedelic and entheogene" and proposes a classification of the substances concerned. The concept of hallucination remains very difficult to define and deserves to be expanded to the "modified states of consciousness" (according to the AUC concept of Anglo-Saxon); which also includes all the experiences related to shamanism, initiative rites and adult transition in traditional societies, as well as those related to coma awakening and OBE (Out of Body Experience). Studying shamanic practices reveals regular use of hallucinogenic mushrooms for collective or individual access to the sacred world and healing. Examples are developed from the use of peyotl in the culture of Huichols Amerindians, the San Pedro cactus, which also contains mescaline and also from the therapeutic use of ayahuasca, experienced in several current research. The Western conception of the therapeutic use of hallucinogens remains associated with the counter-culture of the 1960s, although some experiments (Takiwasi Centre in Peru) are trying to establish a treatment framework for drug addicts.