Preemption is a doctrine under which one law supersedes another law that would otherwise apply. In U.S. constitutional law, the term most often describes federal law superseding state law under the Supremacy Clause of Article VI, which makes the U.S. Constitution, valid federal statutes, and treaties, the supreme law of the land. Valid federal regulations issued under delegated authority can also have preemptive effect, see: Federal Savings & Loan Association et al. v. de la Cuesta, 458 U.S. 141 (1982). Federal law does not automatically replace state law merely because both address the same subject; there must be a recognized basis for the displacement.
Federal Preemption
Congress may preempt state law expressly by including a preemption clause in a statute. Preemption may also be implied. Field preemption exists when Congress intended federal law to occupy an entire regulatory field, leaving no room for state regulation even if the state rule complements federal law. Conflict preemption exists under two scenarios: impossibility and obstacles. Impossibility preemption occurs when compliance with both laws is not possible, and obstacle preemption occurs when a state law is an obstacle to the accomplishment and execution of the full purposes and objectives of a federal law. Courts examine the federal text, structure, and regulatory framework to identify the scope of any displacement.
Whether federal law preempts a state rule depends on the particular federal source and the state law's operation. A detailed federal regulatory scheme does not, by itself, establish field preemption, and overlap between federal and state law is not enough. A court may find only part of a state statute preempted or may limit preemption to particular applications.
In Arizona v. United States, 567 U.S. 387 (2012), the Supreme Court held that federal law occupied the field of alien registration, so even complementary state regulation was barred. The Court also invalidated a separate Arizona provision because it obstructed the federal employment framework. By contrast, in ONEOK, Inc. v. Learjet, Inc., 575 U.S. 373 (2015), the Court declined to find field preemption of state antitrust claims directed at retail natural-gas prices, even though the alleged conduct also affected federally regulated wholesale rates.
In Hencely v. Fluor Corp., 608 U.S. 31 (2026), the Court rejected a categorical rule preempting state tort claims against a military contractor when the challenged conduct was neither ordered nor authorized by the Federal Government. The decision emphasizes that preemption must rest on a valid source of federal law and a recognized basis for supplanting state law, rather than a general federal policy interest standing alone.
State-Local Preemption
Preemption also occurs within a state's legal system. A state constitution or statute may expressly prohibit local regulation, or a state court may find that state law occupies a field or conflicts with a local ordinance. The result depends on the state's constitution, home-rule provisions, statutes, and judicial doctrine. A local ordinance permitted in one state may therefore be preempted in another.
[Last reviewed in August of 2026 by the Wex Definitions Team]