This introduction presents an overview of key concepts discussed in this book. The book focuses on France in the thirteenth century, particularly southern France, where the surviving sources are richest, but also includes some largely neglected evidence for northern France. It discusses the most interesting aspects of the academic study of heresy and inescapability of thinking critically about the sources. The book also focuses on the heretics often called 'Cathars' and 'Waldensians', two groups that demonstrably held some wider appeal in medieval society. Wakefield and Evans focus particularly on heresy, largely ignoring the practical and legal aspects of its repression. The book provides the translations of considerable legislative activity and legal consultation in southern France and two 'guides for inquisitors'. The surviving documentary record reminds the power of the Church, and its determination to crush what it saw as a subversive heretical threat.