This book is concerned less with the substance of heresy than with the charges of heresy and techniques of repression in the archdiocese of Cologne between the early twelfth century and the 1230s. It is based on a close reading of a relatively limited number of sources, including works by Bernard of Clairvaux (whose ‘little foxes’, the author believes, were the heretics in Cologne), Everinus of Steinfeld, Elizabeth of Schönau and her brother Eckbert, Hildegard of Bingen (including the ‘exorcism’ in ms Dendermonde 9, redated here to about 1230), Caesarius of Heisterbach, the Codex of Theoderic of Deutz, the ‘revelations’ of Lepzet of Cologne, and a recently-discovered exemplum in ms Troyes 946, redated 1210-20. No use is made of legal sources or of the Libelli de lite, most of which fall outside the chronological scope of this work. Its conclusions are iconoclastic and in some respects radical. The author argues that before the 1230s there were no widespread movements or general definitions of heresy and no coherent policy of repression. Its emergence in the diocese of Cologne in the early twelfth century was associated with the new religious orders and their criticism of ‘unworthy’ priests, who defended themselves by bringing charges of heresy, often for personal reasons, against the reformers who advocated poverty and the vita apostolica. According to Brunn, heresy before the 1130s was ‘a sub-product’ of the conflict over the establishment of the regular canons (p. 97, cf. p. 131). The ‘discovery’ of heresy was above all the work of Eckbert of Schönau, who in his Liber contra hereses Katarorum, which Brunn dates 1164/5, constructed (largely out of late antique sources, principally Augustine) an apparently coherent doctrine of heresy which he called Catharism. The term ‘Cathar’ in reference to contemporary heresy originated in his works and was rarely used elsewhere. Dualism was mentioned in only two chapters of Eckbert's work, which was largely concerned with the social functions of the Church, such as baptism and marriage. His objectives were to assert the efficacy of good works and clerical mediation. In particular after the Albigensian crusade, however, heresy was transformed from a local challenge into a general threat. In the works of several thirteenth-century writers and in the views of the early inquisitors, especially Conrad of Marburg, who (Brunn argues) was influenced by Caesarius of Heisterbach, heretics were demonised and were said to have worshipped the devil and to have had carnal relations with him in the form of a cat. Alan of Lille, indeed, said that the term Cathar derived from cat. These developments inaugurated a new period in the history of heresy. Brunn's conclusions will doubtless be debated by specialists. They are based primarily on sources from the Rhineland, and the author himself stresses that it remains to be seen whether ‘the Rhenish model’, as he calls it, is applicable to the emergence of heresy in other parts of Europe.