Customary International Law in the 21st Century: Old Challenges and New Debates

This article will survey the new non-traditional scholarship which has emerged in international law to challenge the two long-established sources of customary norms, state practice and opinio juris. With the recent growth, in the international system, of self-contained international criminal tribunals, new challenges facing international law have emerged. Institutionally structured as self-contained legal regimes, international legal tribunals such as the ICTY, ICTR, and now the ICC have nevertheless contributed to a new paradigm within international law. The jurisprudence of these international criminal tribunals, on a wide range of international legal questions, has slowly begun to be elevated into norms of customary international law. Given this fact then, the debate over whether consistent state practice and opinio juris are the only building blocks of customary international law is over, because clearly, for better or for worse, they no longer are. The new question, the new debate, will be over what the implications of this shift in the traditional building blocks of customary international law are, not only on the international system as a whole, but also, surprisingly perhaps, on national (domestic nation state) legal systems as well. The domestic law angle is key, for in the past few years the jurisprudence of these international tribunals has, aside from finding its way into customary international law, also begun to seep into the domestic (mainly criminal) law of several countries.