This is the first book-length study into crusading against Christians, examining this complex phenomenon from the twelfth to fifteenth centuries and across numerous regions, from France to Russia and from southern Italy to the Baltic. Whilst the crusades are an immensely popular topic, those launched against Christian rulers and communities have been comparatively overlooked in the past, with existing studies typically focusing on a particular area, period, or campaign. This volume brings together the expertise of thirteen scholars on a variety of primary and secondary sources not often accessible to Anglophone readership, as well as their knowledge of national discourses which have often shaped historiography. It aims to serve as the first port of call for anyone who wishes to approach crusades against Christians within and without the specialism of crusader studies, and to provide the basis for a thorough comparative analysis of this phenomenon, covering its variety as comprehensively as possible.