This book proposes a new approach to studying the effectiveness of peace operations. It asks not whether peace operations work or why, but how: when a peace operation achieves its goals, what causal processes are at work? By discovering how peace operations work, this new approach offers five distinctive contributions. First, it studies peace operations through a local lens, examining their interactions with actors in host societies rather than their genesis in the politics and institutions of the international realm. In doing so, it highlights the centrality of local compliance and cooperation to a peace operation's effectiveness. Second, the book structures a framework for explaining how peace operations canshape the behaviour of local actors in order to obtain greater cooperation. That framework distinguishes three dimensions of a peace operation's power-coercion, inducement, and legitimacy--and illuminates their effects. The third contribution is to highlight the contribution of local legitimacy to apeace operation's effectiveness and identify the means by which an operation can be locally legitimized. Fourth, the new power-legitimacy framework is applied to study two peace operations in depth: the United Nations Transitional Authority in Cambodia (UNTAC), and the Regional Assistance Mission to Solomon Islands (RAMSI). Finally, the book concludes by examining the implications of this new approach for practice and identifying a set of policy reforms to help peace operations work better. The book argues that peace operations work by influencing the decisions and behaviour of diverse local actors in host societies. Peace operations work better--that is, achieve more of their objectives at lower cost--when they receive high quality local cooperation. It concludes that peace operations are more likely to attain such cooperation when they are perceived locally to be legitimate.