The outsourcing of military and security services is the object of intense legal debate. States employ private military and security companies (PMSCs) to perform functions previously exercised by regular armed forces, and increasingly international organisations, NGOs and business corporations do the same to provide security, particularly in crisis situations. Much of the public attention on PMSCs has been in response to incidents in which PMSC employees have been accused of violating international humanitarian law. Therefore initiatives have been launched to introduce uniform international standards amidst what is currently very uneven national regulation. This book analyses and discusses the interplay between international, European, and domestic regulatory measures in the field of PMSCs. It presents a comprehensive assessment of the existing domestic legislation in EU Member States and relevant Third States, and identifies implications for future international regulation. The book also addresses the crucial questions whether and how the EU can potentially play a more active future role in the regulation of PMSCs to ensure compliance with human rights and international humanitarian law.