This thesis examines how the inquisitio hereticæ pravitatis (the ‘inquisition’) changed in the 14th century. It does so through a comparison of Gui’s Practica inquisitionis heretice pravitatis, and Eymerich’s Directorium inquisitorum, which are the fullest accounts of inquisitorial practice in the fourteenth century. It therefore concentrates on the Languedoc and Aragon, although wider evidence is used where possible. It points out several areas of change between these two points in the evolution of the inquisition: a) Gui’s Practica was a conservative response to a changing heresy threat, which adapted the existing methods of the successful Languedoc inquisition to meet new threats like the Beguins, Pseudo-Apostles and magicians. Eymerich, who came some 50 years later, was a systematizer and an innovator. He defined the inquisition in such a way that any heterodox thinking could be found to be heresy by an inquisitor. He also innovated in seeing the inquisition’s jurisdiction as extending over Jews (not just Jews converted to Christianity and their helpers), blasphemers and magicians. In this he was following secular trends where magicians were increasingly perceived as a problem and blasphemy was considered a serious offence; and in Aragon there had long been efforts amongst Dominicans to get more jurisdiction over Jews. b) Gui worked closely with the secular power and was cautious in his approach to the business of inquisition. Eymerich was more distant from the secular power and envisaged a more autonomous inquisition. c) Although Gui used torture on occasions, Eymerich made torture the default mechanism for resolving nearly all suspected cases of heresy. d) Gui’s inquisition was still a temporary expedient for defined heresies. Eymerich saw the inquisition as a permanent and institutional part of the Church, which would protect it against all doctrinal assault. In this he saw diabolic influences as playing an important role.