The idea of the power or effectiveness of words emerges from the reading of very different sources in the Middle Ages, be it doctrinal texts or practical works. This book is intended to be the broadest possible confrontation on this topic from an intellectual and anthropological perspective. Indeed, in the various cases, in different areas, many cross-cutting issues arose, concerning the elements that were described as determining the effectiveness of speech (the words themselves, rituel, protagonists). Was this effectiveness the subject of a normative narrative? Was it a reflex speech on the part of philosophers or theologians? Were the speaker’s commitment, belief, intent, such as the auditor’s consent or collaboration, decisive factors? Was there an inherent power in the words, or were they merely the vector of a power coming from elsewhere, particularly supernatural? Materials and analyses presented arise from questions which may be of interest to the philosophy of language such as history or anthropology.