This open access book explores the history and historiography of the Heresy of the Free Spirit, the name given to a group of individuals in the late Middle Ages who believed that their spiritual perfection had reached such a state that they were unable to sin. This book explores the historiography of this heresy, revealing how and why an idea of a distinct social movement was constructed and perpetuated across the centuries, from the mid-thirteenth to the twentieth century. It shows how this heretical sect became entangled with both the concept and terminology of another medieval religious movement, the beguines and beghards. Including detailed analysis and contextualization of both medieval inquisitorial sources and modern historiography, the author investigates how historical interpretations of this so-called sect have been influenced by the methods and practices of historians from the eighteenth to the twentieth century.