United Nations peace operations have undergone multiple transformations over the more than seventy years of their existence. Multidimensional peace operations have organised elections, helped deliver humanitarian assistance, advised on army and police reform, and fought rebel groups. Such operations not only represent a core pillar of the multilateral peace and security architecture but also reshape the lives of millions of people around the world. This volume provides the first comprehensive overview of multiple theoretical perspectives, offering examples of how International Relations theories apply to specific policy issues and demonstrates how major debates on UN peace operations - on civilian protection, local ownership, or gender mainstreaming - benefit from theoretical exploration. With insightful contributions from international academics, this is an essential book for scholars, students, and experts working on peace and security and international cooperation.