This book explores how ayahuasca becomes an object of public controversy, contestation, and governance across diverse sociopolitical contexts. Rather than seeking a universal or essential definition of ayahuasca, the research follows the ways in which it is made legible, contested, and authorized in interactions involving state institutions, religious groups, Indigenous organizations, scientists, and activists. Drawing from the anthropology of secularism, pragmatist approaches to public problems, and the notion of “public controversies,” the book approaches religion not as a pre-given category, but as something historically produced through disputes over legitimacy, tradition, science, and law. Anchored in ethnographic research across Brazil, the United States, and France, the introduction outlines the methodological and theoretical orientation of the book, emphasizing a shift from fixed categories to the dynamic processes through which ayahuasca acquires meaning and legal status. It presents the main research questions, conceptual frameworks, and structure of the work, setting the stage for an analysis of the entangled moral, political, and epistemological dimensions that shape the contemporary regulation and use of ayahuasca.