Private military and security companies play an increasingly central role in national and international contexts. Private companies train armed forces, they guard military and civilian installations, they provide intelligence, and they back up military and police operations. This book is about the fundamental question what the (re-)emergence and expansion of the private sector into military and security matters means for state authority over the use of force. By placing the current developments of the sector in historical perspective and giving a picture of the role played by private firms in military and security matters at present, this book shows that private involvement in military affairs is neither novel nor undermining states in any simplistic way. Rather the emergence of the private market for force is reshaping state authority. This book explains how and why.