The preemption doctrine derives from the Supremacy Clause of the Constitution which states that the "Constitution and the laws of the United States...shall be the supreme law of the land...anything in the constitutions or laws of any State to the contrary notwithstanding."  This means of course, that any federal law--even a regulation of a federal agency--trumps any conflicting state law.

Preemption can be either express or implied.  When Congress chooses to expressly preempt state law, the only question for courts becomes determining whether the challenged state law is one that the federal law is intended to preempt.  Implied preemption presents more difficult issues, at least when the state law in question does not directly conflict with federal law.  The Court then looks beyond the express language of federal statutes to determine whether Congress has "occupied the field" in which the state is attempting to regulate, or whether a state law directly conflicts with federal law, or whether enforcement of the state law might frustrate federal purposes.

Federal "occupation of the field" occurs, according to the Court in Pennsylvania v Nelson (1956), when there is "no room" left for state regulation.  Courts are to look to the pervasiveness of the federal scheme of regulation, the federal interest at stake, and the danger of frustration of federal goals in making the determination as to whether a challenged state law can stand.

In Silkwood v Kerr-McGee (1984), the Court, voting 5-4,  found that a $10 million dollar punitive damages award (in a case litigated by famed attorney Gerry Spence) against a nuclear power plant for negligently allowing employee (and union activist) Karen Silkwood to be contaminated with plutonium was not impliedly pre-empted by federal law.  Even though the Court had recently held that state regulation of the safety aspects of a federally-licensed nuclear power plant was preempted, the Court drew a different conclusion with respect to Congress's desire to displace state tort law--even though the tort actions might be premised on a violation of federal safety regulations.

Cipollone v Liggett Group (1992) was a closely-watched case concerning the extent of an express preemption provision in two cigarette labeling laws of the 1960s.  The case was a wrongful death action brought against tobacco companies on behalf of Rose Cipollone, a lung cancer victim who had started smoking cigarette in the 1940s.  The Court considered the preemptive effect on state law of a provision that stated "No requirement based on smoking and health shall be imposed under state law with respect to the advertising and promotion of cigarettes."  The Court concluded that several types of state tort actions were preempted by the provision, but allowed other types to go forward.  Perhaps the most interesting aspect of the case, from a constitutional standpoint, is the debate between justices over whether express preemption provisions should be read narrowly (a view adopted by seven justices) or read normally (a view favored by Justices Thomas and Scalia).

In American Insurance Association v Garamendi (2003), the Court considered the constitutionality of a California law designed to help California Holocaust survivors collect on unpaid insurance claims from German insurance companies.  Despite the absence of any clear statement in any executive agreement preempting state laws such as that of California, the Court, voting 5 to 4, found that the state Act conflicted with national policy and "stands in the way of [the President’s] diplomatic objectives.” The four dissenters included both liberals (Ginsburg and Stevens) and conservatives (Scalia and Thomas).  The dissenters argued, "Courts step out of their proper role when they rely on no legislative or even executive text, but only on inference and implication, to preempt state laws on foreign affairs grounds."

Gonzales v Oregon (2006) considered whether Congress, in enacting the Controlled Substances Act, intended to pre-empt state laws such as that of Oregon's which authorized physicians (under strictly controlled circumstances) to prescribe lethal doses of controlled drugs for terminally ill patients.  (The case also raised the administrative law issue of whether the Attorney General acted within the scope of his statutory authority when he issued regulations criminalizing the prescription of lethal drugs by physicians.)  The Court, ruling 5 to 4, held that the Act did not authorize pre-emption of Oregon's Death with Dignity Act.