Editor's Summary: Last year, Tony Blair and Arnold Schwarzenegger signaled their commitment to join the United Kingdom and California in efforts to combat climate change. In this Article, Hannah Chang examines whether California can legitimately join its carbon market to the European Union's emissions trading scheme. She sets forth the foreign affairs federalism considerations that California must address and points to elements of its transnational action that ought to persuade a court to uphold its state legislation in the face of a foreign affairs preemption challenge. She ultimately concludes that California's legislation might well be foreign affairs-preempted, but that this result ought not be inevitable given the unique aspects of California's actions that call for a rethinking of foreign affairs federalism.