If there is one branch of contemporary political theory that has many concrete examples of ‘evil’ as its subject matter, it is international political theory. Indeed, the word has made a spectacular return to international political discourse since 9/11. In this chapter I suggest a novel approach to theorising ‘evil’ in international political theory. It is an approach that diverges from much of the established (and very useful) commentary that has explored the concept as it emerges in the rhetoric of political leaders on both sides of the war on terror. I think that dealing with evil in that context has limited political utility and suggest that we turn our attention to the broader institutional construction of, and response to, evil in international society. As a sub-discipline, that has traditionally had war as its principal focus normative International Relations (IR), theorists have had to deal not just with the prospect of armed conflict between states but with genocide, war crimes and crimes against humanity, including the killing of civilians, institutionalised rape and torture and the grotesque human suffering that follows conflict-created famine and refugee flows. At the extreme end of this suffering is a phrase that has enormous significance in international politics. That phrase is found in the claim that there are ‘acts that shock the conscience of mankind’. This phrase has been used time and time again (sometimes with the more politically correct ‘humankind’ as its referent) to evoke great evil of such unimaginable proportions that it precipitates new forms of legal, political or military sanction.