This work examines the history of private military forces, the types of private armed forces, the conflict environments in which private forces operate, and approaches to the study of the privatization of security. Through an array of theoretical approaches and empirical material, this surveys private armed forces and directly challenges conventional stereotypes of security contractors. Private armed forces are nothing new. The estimated ratio of contractor to military personnel during the American Revolution was about 1 to 6. Yet the use of private military companies has proliferated exponentially during the 21st century, and today, they are largely seen as a threat to global security. But are they dangerous or a necessary adjunct to Department of Defense forces? This provides a comprehensive survey of the private military groups involved in conflicts worldwide. Organized around four themes, it covers the history of private military forces since 1600, the main contemporary actors and their defining characteristics, the environments in which private armed forces operate, and provides an analysis of the logic behind privatizing security. This book goes beyond conventional knowledge, offering both a theoretical approach and a new, practical perspective to advance the understanding of the ongoing climate of global instability and relevant players within it. Numerous examples help the reader grasp the full range of real world challenges and conceptual facets surrounding this polarizing topic.