Common accounts of federalism and preemption do not envision states as supreme. According to the conventional view, states may share regulatory space with the federal government, carry out federal programs, or operate independently and even exclusively. But in two provisions in federal law, states play a role of supremacy: they are given extraordinary power – what I call ‘reverse preemption’ – to veto federal agency decisions that conflict with state policy. One provision allows states to override federal actions that conflict with state coastal management; another gives states the power to reverse or condition federal licensing decisions that impede state water quality goals. This chapter examines these reverse preemption provisions and explores their ramifications. It suggests that reverse preemption provisions enhance the power of states with strong environmental preferences through an unexpected form of federalism. The chapter describes the ways in which the provisions allow Congress to check executive branch decision-making that could undermine environmental protection, particularly when Congress and the Presidency are held by different political parties. Thus, reverse preemption provisions play an important function in bolstering separation of powers values. Finally, the chapter includes preliminary suggestions for the use of reverse preemption provisions in other policy areas.