The contemporary world is beset with a wide variety of conflicts, all of which have features without historical precedent. While most accounts of peacekeeping focus on attempts to limit violent conflict, this traditional view hardly captures the variety of challenges that today’s peacekeepers face. Peacekeepers are now thrust into the unconventional roles of monitoring elections, facilitating transitions to the rule of law, distributing humanitarian aid, and resolving conflicts in civil societies that are undergoing transformation. This is the context for understanding the activities of modern-day peacekeepers. In When Peacekeeping Missions Collide, Paul F. Diehl, Daniel Druckman, and Grace B. Mueller provide an original and comprehensive assessment on how different peacekeeping missions intersect with one another in contemporary conflicts. They begin by documenting the patterns of peacekeeping missions in 70 UN operations, noting the dramatic increase in number and diversity of operations since the end of the Cold War as well as the shift to conflicts with a substantial internal conflict component. They then turn to the overarching question of the book: how do individual peacekeeping missions impact the outcomes of other missions within the same operations? To answer this, the authors have developed a novel dataset of UN peace operations from 1946-2016 to assess mission compatibility. Moreover, the authors utilize five detailed case studies of UN peacekeeping operations featuring mission interdependence and then measure the results against their theoretical expectations. Ultimately, the model they have developed for analyzing the effectiveness of the far more complex peace operations of today–relative to the simpler operations of the past–is essential reading for scholars of peacekeeping and conflict management.