Climate change policy analysis has focused almost exclusively on national policy and even on harmonizing climate policies across countries, implicitly assuming that harmonization of climate policies at the subnational level would be mandated or guaranteed. We argue that the design and implementation of climate policy in a federal union will diverge in important ways from policy design in a unitary government. National climate policies built on the assumption of a unitary model of governance are unlikely to achieve the expected outcome because of interactions with policy choices made at the subnational level. In a federal system, the information and incentives generated by a national policy must pass through various levels of subnational fiscal and regulatory policy. Effective policy design must recognize both the constraints and the opportunities presented by a federal structure of government. Furthermore, policies that take advantage of the federal structure of government can improve climate governance outcomes.