The main aim of this book is to argue that the use of private force by states has been restricted by a norm against mercenary use. The book traces the evolution of this norm, from mercenaries in medieval Europe through to private security companies in modern day Iraq, telling a story about how the mercenaries of yesterday have evolved into those of today in the process. The norm against mercenaries has two components. First, mercenaries are considered to be immoral because they use force outside legitimate, authoritative control. Second, mercenaries are considered to be morally problematic because they fight wars for selfish, financial reasons as opposed to fighting for some kind of larger conception of the common good. The book examines four puzzles about mercenary use, and argues that they can only be explained by understanding the norm against mercenaries. First, the book argues that moral disapproval of mercenaries led to the disappearance of independent mercenaries from medieval Europe. Second, the transition from armies composed of mercenaries to citizen armies in the nineteenth century can only be understood with attention to the norm against mercenaries. Third, it is impossible to understand why international law regarding mercenaries, created in the 1970s and 1980s, is so ineffective without understanding the norm. Finally, the disappearance of companies like Executive Outcomes and Sandline and the development of today's private security industry cannot be understood without the norm. This book is a project of the Oxford Leverhulme Programme on the Changing Character of War.