Many ceasefires and peace agreements in civil conflict are initially unsuccessful. Some give way to renewed, and often escalating, violence. In other cases, peace processes have become interminably protracted: lengthy and circular negotiations in which concessions are rare. Given the huge material and human costs of a failed peace process, the international community has a strong interest in helping these processes succeed and addressing threats to their implementation. Challenges to Peacebuilding approaches this problem by focussing on'spoilers': groups and tactics that actively seek to obstruct or undermine conflict settlement through a variety of means, including terrorism and violence. It considers why spoilers and spoiling behaviour emerge and how they can be addressed, drawing upon experience from Northern Ireland, the Basque region, Bosnia, Colombia, Israel-Palestine, Cyprus, the Caucasus and Kashmir. This volume takes a'critical'approach to the concept of spoiling and considers a broad range of actors as potential spoilers: not only rebel groups and insurgents, but also diasporas, governments, and other entities. It also demonstrates that ill-conceived or imposed peace processes can themselves sow the seeds of spoiling.