The most famous event of the thirteenth century war against the Cathars of the Languedoc is an alleged command given by an Abbot "Kill them all! God will know His own". Almost everyone has heard of it, even if the context is a little hazy. For well over six hundred years the claim - made by a sympathetic contemporary clerical writer - was accepted as fact. But historians over the last hundred years have been more sceptical, often describing the story of the command as "apocryphal". Almost no original work on the question has been carried out by modern historians in the English-speaking world, and only a little by Francophone historians. This monograph aims to provide a thoroughgoing reassessment of the question. It presents a mass of new evidence, exposes previously unrecognised significance in existing evidence, and frames new arguments that will undoubtedly shift received opinion among historians.