Based on extensive study of the primary and secondary sources, Damian J. Smith here provides the first full account of the combined influence of crusade, heresy and inquisition in and about the lands of the Crown of Aragon until the death of James I the Conqueror in 1276. This work deals with the gradual loss of influence of the Crown in Provence and Languedoc culminating in the treaty of Corbeil in 1258. It then investigates the extent of heresy in the lands of the Crown and in other areas of Christian Spain. In the final part, the origins and development of the Aragonese inquisition are discussed in detail with a particular emphasis on the role of Ramon de Penyafort.