Abstract: Constitutional law relies on the diffusion of powers among different individuals in different institutions to produce many desirable institutional goods: checks and balances, democratic accountability, and effective government, for instance. Federalism and the separation of powers have been presented as the primary institutional arrangements generating this diffusion. Scholars and jurists alike, though, have largely neglected another form of diffusion: federal decentralization. Federal power cannot be appropriately diffused if it is geographically concentrated in a single place. Federal decentralization ensures that federal officials in places distant and therefore different from Washington compete with and constrain federal officials in Washington. This Article identifies and evaluates federal decentralization as a dimension of constitutional law. This Article first uncovers the long but lost history of federal decentralization, and places it at the core of our constitutional experience from the Founding to its current moment on constitutional center stage. The First Congress located important federal officials in a different metropolitan area than the President and Congress, and arranged for the Congress and the White House to operate in different buildings in different neighborhoods. The current Congress has considered legislation proposed by both parties that would increase federal decentralization. This Article then argues that federal decentralization makes visible the diffusions of power that federalism and separation of powers cannot provide and, executed properly, attempts to provide them. It gives federalism the voice it needs and separation of powers the exit it lacks. Federalism aspires to empower local majorities, and federal decentralization enhances the voice of local majorities by making them empowered neighbors rather than unfamiliar strangers to federal officials—and even permits local majorities to act as federal officials themselves. The separation of powers aspires to generate rivalrous branches, but rival interests can only be generated by ensuring that sometimes federal officials exit Washington rather than operate in it. Federal decentralization, though, risks injecting excessive diffusion into the American system. It therefore requires its own vocabulary to recognize and resolve the persistent set of institutional design challenges that it raises.