Introduction This research describes and analyses Upper Amazon shamanic practices around the ceremonial use of ayahuasca in Spain. The multi-sited fieldwork was done between August 2008 to August 2014, primarily in Spain, and secondarily in Peru. The description focuses on the activity of a group of shamans, some of Spanish origin and some native to the Amazon and Andean regions, during ayahuasca ceremonies. This activity aims at healing the participants in the ceremonies, and is analysed as an alternative rationality to the dominant rationality in our society based on a dualistic ontology. This alternative rationality shows a congruence between means used by shamans and the intended therapeutic outcome. Shaman is defined as a person who has the capacity to interact with the world of ‘spirits’, and to mediate between those ‘spirits’ and a human group. The term shaman is used in this research as an analytical category for cross-cultural study on the activity of these individuals and not as a self-designation made by themselves, that is no the case. Objectives and Methods The research hypothesis is that the shamans alternative rationality is based on a coherent episteme consisting of three basic elements: (a) their energetic or spiritual conception of reality; (b) their spritual agency, that is acquired through a specific training process called ‘diet’; and (c) the ethical intentionality in their interaction and mediation between the ‘spirits’ and a human group. Thus, the Thesis main objective is the description and analysis of these three elements, and their therapeutic outcome. The method used in the fieldwork was participant observation and two of its variants: collaborative participation and radical participation. Radical participation is the method used to interpret, describe and analyse the work of shamans during ayahuasca ceremonies. This method is defined as the means to experience what informants are experiencing during ceremonies. The use of radical participation is justified theoretically in two ways: in the so-called ontological turn and its ‘taking informants seriously’ leitmotiv, and the decolonial turn which advocates a decolonization of knowledge that entails a decolonization of the being and, consequently, its transformation. Results In order to understand the meaning given by shamans to their practises and the ontological foundations on which those practises are based, it is necessary to develop an ad hoc theoretical framework. This framework seeks to avoid the symbolic interpretation of the shaman interaction with ‘spirits’ and to establish a radical distinction between religion and spirituality. A conceptual framework is proposed based on the following principles: (1) to diffferentiate between (religious) beliefs and (spiritual) knowledge; (2) to conceive spiritual practises as asymbolic practises; (3) to develop the concepts of ceremony and ceremonialism as an alternative to the concept of ritual; (4) to distinguish the shaman from the priest due to their ontological and paradigmatic differences. It is also relevant for the analysis of shamanic practices to distinguish between the notions of shaman and shamanism introducing the idea of shamanhood to refer, exclusively, to the individual dimension and to use shamanism when alluding to the collective dimension of shamanic practises. It is necessary to improve a conceptual framework not dependent on a biomedical perspective in order to analyse the practises of shamanic healing. To this end, this research proposes the use of holistic concepts to analyse holistic phenomena: notions as healing, misfortune or affliction, and healing-affliction processes are preferable to those of health, disease, and health-illness processes. Ayahuasca ceremonies are described in Chapter 5. A diet of tobacco taking place in Iquitos (Peru) is described in Chapter 6. This diet is included as a microethnography to be also used as an extended case in the analysis of the ethnography in Chapter 7. Conclusions The analysis of the ethnography concludes that from the shaman’s point of view the difficulty of their activity does not lie in their interaction with an invisible spiritual world, but to act for the good of the participants. Shamans, and ‘spirits’, ‘forces’ or ‘energies’ may be classified according to the effects caused in participants of ayahuasca ceremonies and, therefore, may be described as possessing a certain ethical polarity. The positive therapeutic outcome of ayahuasca ceremonies may be understood as a set of gradual and incremental healing-affliction processes. Those processes as a whole are called a meta-process of spiritualization which entails a transformation of participant’s worldview. Finally, the research proposes an extrapolation from the study of Upper Amazon shamanic practises to the study of various therapeutic and spiritual practises associated with the New Age and Complementary and Alternative Medicine. Thus, in order to study non-human entities from an Anthropology of Spirituality perspective, the research suggests some meta-cultural basis which would allow a better comprehension of health and spirituality changing values currently operating in our society.