The concept of heresy has played a major role across Christian history. Traditionally, heretical sects have been regarded as distinct, real-life groups of people who had departed from the stable orthodox traditions of Christianity and who posed a threat that needed to be addressed, sometimes through violent repression. More recently, scholarship has focused on the notion of heresy as discourse, placing particular emphasis on its literary construction and the social and cultural contexts in which it was deployed. This literature has generated significant debates about the nature and historicity of many heresies. The Cambridge Companion to Christian Heresy provides a systematic and up-to-date guide to the study of this topic and its methodological challenges. The opening chapters explore different forms of written material that have played vital roles in historical disputes and in modern scholarly accounts. These are followed by case studies of thirteen notable heresies, ranging from the Gnostics through to the Hussites at the dawn of the Reformation.