Climate change has been a key arena for the development of transnational environmental law because it is a global problem with complex local linkages. More so than many other environmental regimes, there has been a proliferation of activity by cities and other subnational actors in the climate change space. This flurry of activity takes on a transnational dimension when, for example, subnational governments participate in cross-border networks to develop and implement norms, practices and voluntary standards. Subnational governments have also become involved in litigation. In this context, we have seen some subnational governments proactively pursuing litigation to seek progress on climate change action where national governments and businesses are perceived to be taking insufficient action to address the problem. This chapter explores the role of subnational entities - cities, states, and regions - in developing and implementing transnational climate change law. It advances the argument that the participation of subnational actors in transnational legal processes invites us to reexamine theories of international lawmaking that posit the state as the only legally relevant actor in international affairs.