The Inquisition played a central role in European history. It moulded societies by enforcing religious and intellectual unity; it helped develop the judicial and police techniques which are the basis of those used today; and it helped lay the foundations for the persecution of witches. An understanding of the Inquisition is therefore essential to the late medieval and early modern periods. This book looks at how the philosophy and practice of Inquisition developed in the fourteenth century. It saw the proliferation of heresies defined by the Church (notably the Spiritual Franciscans and Beguines) and the classifcation of many more magical practices as heresy.The consequential widening of the Inquisition's role in turn led to it being seen as an essential part of the Church and the guardian of all the Church's doctrinal boundaries; the inclusion of magic in particular also changed the Inquisition's attitude towards suspects, and the use of torture became systematised and regularised. These changes are charted here through close attention to the inquisitorial manuals of Bernard Gui and Nicholas Eymerich, using other sources where available. Gui's and Eymerich's personalities were important factors. Gui was a successful insider, Eymerich a maverick, but Eymerich's work had the greater long-term influence. Through them we can see the Inquisition in action. DEREK HILL gained his PhD from the University of London.