The SUPREMACY CLAUSE of the Constitution (Article VI, clause 2) requires that inconsistent state laws yield to valid federal laws. Preemption is the term applied to describe invalidation of state laws by superior federal law.
Strictly speaking, the issue of preemption is not one of constitutional law. The issue is not what Congress has the power to do, but what Congress has done. Where Congress has made an articulate decision whether particular state laws should survive a new scheme of federal regulation, the issue is settled. For example, in enacting minimum federal standards for automobile pollution control equipment in 1967, Congress prohibited states from enforcing more restrictive standards but made an exception for the State of California. There has been no need for litigation to mark the contours of preemption in that context. Insofar as there is a "doctrine" of preemption, it concerns the treatment of preemption by federal laws where Congress has ignored the issue.
Since preemption cases theoretically turn on construction of federal statutes to determine whether Congress intended to preempt state laws, there are limits to generalizations that can be drawn from the decisions. Each case construes a federal statute with a distinct regulatory structure and legislative history. It is particularly difficult to classify the simplest form of preemption cases?those where the claim is made that the terms of federal and state law are flatly inconsistent. Federal law may, for example, give express permission to engage in conduct prohibited by state law. An early famous case of this type was GIBBONS V. OGDEN (1824).
The most complex issues of preemption arise where it is concededly possible to comply with mandates of both state and federal law. The question then arises whether Congress intended to "occupy the field," or whether the challenged state law's enforcement would interfere inordinately with the policies of the federal law. State law may provide additional sanctions for conduct prohibited by federal law. (In California v. Zook, 1949, the Court sustained a state law that punished interstate motor transport operating without a federal permit.) State law may impose more stringent regulations than federal law. (In Napier v. Atlantic Coast Line R.R., 1926, the Court held that a state law requiring railroad safety equipment was preempted by a federal law that required less equipment.) Finally, it may be argued...