In the United States, law and policy always function within layers of government authority – federal, state, and local. This book primarily examines the choices policy makers, legislatures, or agencies face in allocating and coordinating responsibility among these layers; choices that will always be constrained by any limits placed on them by the U.S. Constitution, as interpreted by the Supreme Court. This chapter explores the doctrine that the Supreme Court has elaborated to address one set of those constraints, namely the doctrine of preemption.

Preemption doctrine rests on Article VI of the Constitution, which provides that the laws and treaties of the United States “shall be the supreme law of the land … anything in the Constitution or Laws of any State to the contrary notwithstanding.” When is a state or local law “contrary” to the laws or treaties of the United States? The doctrine of preemption seeks to answer that question, in all the various contexts in which it may arise. Although major elements of the doctrine have been clearly delineated by the Supreme Court, the outcomes of specific clashes between federal and state law prove to be far from certain. The beginning sections of this chapter examine the stated doctrine and some of its difficulties.