The issue of prisoners in war is a highly timely topic that has received much attention from both scholars and practitioners since the start of the military operations in Afghanistan and Iraq and the ensuing legal and political problems concerning detainees in those conflicts. This book analyzes these contemporary problems and challenges against the background of their historical development. It provides a multidisciplinary yet highly coherent perspective on the historical trajectory of legal and ethical norms in this field by integrating the historical analysis of war with a study of the emergence of the modern legal regime of prisoners in war. In doing so, it provides the first comprehensive study of prisoners, detainees and internees in war, covering a broad range of both regular and irregular wars from the crusades to contemporary counterinsurgency campaigns. The book revolves around two major developments: First, there has been a continuous increase in the political relevance of prisoners in war, in particular since the emergence of POW camps in the nineteenth century. Secondly, and related, the growth in the legal regime pertaining to prisoners had contradictory consequences. Whilst it enhanced the protection of prisoners in regular conflicts, its state-centric bias tends to exclude combatants who do not fit the template of regular inter-state war. Detainees in the 'war on terror' embody both tendencies, the development of which, however, is by no means a novel phenomenon. This book is a project of the Oxford Leverhulme Programme on the Changing Character of War.