Over the last two decades, the international community has taken on the task of rebuilding societies in the aftermath of mass-atrocities. Through a combination of trial and error and vigorous academic research, a relatively clear (and semi-malleable) blueprint of post-atrocity peacebuilding has developed. This includes setting up a temporary international transitional authority, establishing democracy, facilitating economic development, and holding war crime trials. Though there are volumes of studies which address the pragmatic strengths and weaknesses of these key elements of peacebuilding, to date political theorists have not critically analyzed the moral legitimacy of these policies. My thesis aims to fill this gap. The overarching question of this thesis is this: What moral duties does the international community have to post-atrocity societies? To answer this question, I critically examine the normative issues involved in the four key aspects of peacebuilding (identified above). Using the framework of just war theory and a cosmopolitan theory of fundamental human rights, I argue that, in most post-atrocity cases, the international community has duties to remove atrocity-committing regimes from power, occupy the target-state and act as a transitional authority, help facilitate the creation of democracy and economic development, and hold war crimes trials. These duties, of course, are extremely complicated and limited and these qualifications are examined and developed throughout. Running through the construction of my theory of post-atrocity duties is a clear message: we – the international community – have obligations to the victims and survivors of atrocities. In other words, providing assistance in the wake of mass-atrocities is not a supererogatory act of charity, rather, it is a duty which we owe to the victims of these horrible crimes.