The state role in environmental regulation and enforcement varies significantly across the spectrum of environmental issues and programs. In some environmental programs, particularly those involving regulation of nationally marketed commodities, Congress has left the states no regulatory role: Federal statutes wholly preempt state regulation and enforcement, leaving policymaking, standard setting, and enforcement entirely in federal hands. More commonly, the states are junior partners in joint federal-state regulatory enterprises, where the federal government defines national environmental policy and sets standards (although it may allow the states to adopt stricter standards), and federally approved state agencies issue and enforce permits subject to federal oversight. Finally, in a few environmental programs, the federal government has permitted and even encouraged the states to adopt parallel regulatory programs. A few federal environmental programs, particularly those directly regulating nuclear waste and nationally marketed products often leave little or no room for state environmental policies and preferences. For example, under the Clean Air Act, the national government defines, monitors, and enforces the air pollution standards for new car; states have no regulatory role until the car is sold to a consumer. The reason is plain: It would be hugely inefficient to allow fifty states to set their own emission standards. Even here, however, Congress offers states a modest policy making role; states may adopt the "California standards," which are stricter than the national standards. A few Northeastern states (as well as California) have done so. &nbs ...