Since the end of the Cold War and the emergence of “asymmetric” threats like terrorism, the military has been increasingly entrusted with tasks traditionally belonging to the police. This development is visible through the new challenges posed to modern Peace Support Operations (PSO), intended as an umbrella definition covering different types of post-conflict peace operations, be these mandated under Chapter VI or Chapter VII of the United Nations Charter, with either peace-keeping, peace-enforcing or even peace-building goals. The aim of this volume is primarily to provide guidance, in the format of a handbook, to those deployed in the field and who are confronted with legal issues. In order to achieve this goal, the handbook is structured as follows: after this introduction, Part II addresses the general question whether law enforcement shall be a PSO task. Law enforcement is perceived by some states as a matter of self-defense.Part III then addresses the limits and possibilities of law enforcement by PSO.The discussion continues with Part IV, which provides some practical tools for those deployed to the field. Part V focuses instead on law enforcement within PSO, illustrating problems related to the prosecution of members of PSO forces suspected of illegal activities, and Part VI then draws the conclusions.