Heresy was a recurrent problem for the established church throughout the middle ages, and it is here examined in the context of the medieval duchy of Aquitaine. The author traces forms of dissent there back to the influence of Balkan dualism, indicating the vast spread of heretical ideas throughout Europe. She goes on to offer an account of Catharism in north-western Languedoc, using neglected evidence for its reception and rejection by the families and towns of the county of Agen to shed light on heretical adherence in the Languedoc more widely, in peace-time, during the Albigensian Crusade, and under the Inquisition. Dr CLAIRE TAYLOR teaches in the Department of History at the University of Nottingham.