Cathars have traditionally been regarded as the most organised challenge to orthodox Catholicism in the medieval West; historians have considered their set of beliefs, inspired by Balkan dualism, as the most radical among medieval heresies. However, this paradigm has been recently challenged by an alternative view, according to which Catharism was a construct of its persecutors and its supposed radical views are no more than a myth. This volume, bringing together some of the most distinguished international scholars in the field, aims to continue the debate and to open up new areas for reseach. Focussing on duality and anti-materialist beliefs in southern France, northern Italy and the Balkans, it considers a number of crucial issues. These include what constitutes popular belief; how (and to what extent) societies of the past were based on the persecution of dissidents; and whether heresy can be seen as an invention. At the same time, the essays shed new light on some key aspects of the political, cultural, religious and economic relationships between the Balkans and more western regions of Europe in the Middle Ages.