Over the past twenty years, private military companies (PMCs) have become significant elements of national security arrangements, assuming many of the functions that have traditionally been undertaken by state armies. The emergence of PMCs reflects changes wrought by broad geo-political forces, in particular the end of the Cold War, the widespread emergence of internal conflicts throughout the developing world, and the triumph of neo-liberalism among the policy elites of many Western nations. They have rapidly become big business: according to some estimates, they have been the most rapidly growing sector of the international economy over the past decade with an annual turnover well in excess ofUS100 billion. This book provides an interdisciplinary overview of PMCs: what they are, why they have emerged in their current form, how they operate, their current and likely future military, political, social and economic impact, and the moral and legal constraints that do and should apply totheir operations. It is divided into two sections: normative/ethical issues raised by the development of PMCs function and regulation, and their relationships with other institutions. Private Military Companies will be of much interest to students of private military companies, international security and politics and international relations.