Many ceasefires and peace agreements in civil conflict are initially unsuccessful. Whilst some give way to renewed and often escalating violence, others have become interminably protracted leading to lengthy 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. This publication focuses on the groups and tactics (referred to as 'spoilers') that actively seek to obstruct or undermine conflict settlement through a variety of means, including terrorism and violence, drawing upon experience from Northern Ireland, the Basque region, Bosnia, Colombia, Israel/Palestine, Cyprus, the Caucasus and Kashmir. It explores the concept of 'spoiling' and spoiling behaviours from a broad range of interests involved (including rebel groups and insurgents, diasporas and governments), considers how this can be addressed, and demonstrates how ill-conceived or imposed peace processes can themselves contribute to the problem.