This book brings together a rich and diverse range of medieval sources to examine key aspects of the growth of heresy and dissent in southern France in the twelfth and thirteenth centuries and the Church's response to that threat through the subsequent authorisation of the Albigensian Crusade. The reader is introduced to themes which are crucial to our understanding of the medieval world: ideologies of crusading and holy war, the complex nature of Catharism, the Church's implementation of diverse strategies to counter heresy, the growth of the papal inquisition, southern French counter-strategies of resistance and rebellion, and the uses of Latin and the vernacular to express regional and cultural identity. This timely and highly original study not only brings together previously unexplored and in some cases unedited material, but provides a nuanced and multi-layered view of the religious, social and political dimensions of one of the most infamous conflicts of the High Middle Ages.