Subnational implementation of international law has always been a part of the international legal system. States rely on their own legal and political systems to make international agreements and to give effect to international legal norms and obligations. What invites focused attention to the practice now is 1) the growing prominence of such activity in areas like human rights and climate governance and 2) the emergence of direct engagement by subnational units with each other across international borders without going through their national authorities or governments. These developments warrant attention because of their implications for international law as a governing system.

In a global world, subnational activity can be seen as a positive development to increase international law’s ability to reach those most directly affected by international norms and standards. At the same time, subnational practices might create problems of coherence within a normative regime and complicate the ability of states to be held accountable for the actions of jurisdictions for which they are ultimately responsible. Not only are subnational units now visibly active in international fora, they may pursue policies that differ from their national governments and may do so by joining forces with similarly situated jurisdictions in other countries. These practices will not go away and will likely only develop further and intensify. The international law governance question is how should/can international law as a legal system respond, This chapter considers some approaches to this question.