This Article poses a question about deference that remains surprisingly unresolved: when Congress delegates to both state and federal agencies under a “cooperative federalism” scheme, who gets deference when interpreting that law, the state or federal agency? This question has special significance for energy and environmental law because of how common cooperative federalism is to those fields. The Article discusses a recent series of challenges relating to the Public Utility Regulatory Policies Act that pose this question, and presents an answer: courts should consider whether Congress chose “federalism” or “decentralization,” and deference should operate differently depending on that choice.