The institution of crimes against humanity at Nuremberg in 1945 was an event which marked the birth of cosmopolitan law as a social reality. Cosmopolitan law has existed as an abstract idea at least since the writings of Kant in the late eighteenth century, but Nuremberg turned the notion of humanity from a merely regulative idea into a substantial entity. Crimes against humanity differ significantly from the traditional categories of international law: war crimes and crimes against peace. While the latter (...) generally treats states as subjects of right and upholds the principle of national sovereignty, the former treats individuals as subjects of right and encroaches on national sovereignty. I focus here on the writings of Hannah Arendt on the Nuremberg trials, the Eichmann trial and the actuality of crimes against humanity perpetrated in totalitarian regimes. I explore her relation to the optimistic and naive cosmopolitanism of Karl Jaspers, as well as to the two prevailing forms of critical thought: the cynical realism towards the law adopted by many of the defendants, including Carl Schmitt, and the deconstructive attitude to humanism and technology adopted by Martin Heidegger. I maintain that Arendt's rugged cosmopolitanism, tested against its critics, remains exceptionally revealing on the issue of crimes against humanity now that this issue has re-emerged, after many years of silence, as arguably the most important question of our own day. (shrink)