Preemption

"This Constitution, and the Laws of the United States which shall be made in Pursuance thereof ... shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding." Art. VI, Par. 2

There, in a nutshell, is preemption. State laws are "Contrary" to federal law when in conflict either with the constitution (preclusion) or with federal law (preemption).

State laws can conflict with federal law in several ways:

Express Preemption: Congress explicitly states its intent to foreclose state regulation

Implied Preemption: Congress is silent on the face of a law, as to preemption, but the nature of the law is such as to foreclose state regulation. Implied preemption comes in two flavors:

Conflict: 1) it is physically or logically impossible for the object of regulation to comply with both federal and state law simulteneously; 2) state law stands as an obstacle to the full accomplishment of congress' purpose.

Field: federal regulation has so dominanted a field as to "occupy it," leaving no room for supplemental state regulation.

But, finding either express or implied preemption doesn't end the analysis. The challenged state law must be within the scope of that preemption. Determining the preemptive scope of a federal law is often harder than answering the abstract question of whether congress has explicitly or implicitly preempted state law.