Definition
Preemption is the doctrine that, when valid federal law and state law conflict, the federal law prevails. It flows from the Supremacy Clause of Article VI, which makes the Constitution and federal laws the supreme law of the land.
Preemption does not give the federal government unlimited power. It applies only where Congress has acted within its constitutional authority and where state law genuinely clashes with that federal action.
Why it matters
How it works
Courts recognize a few forms of preemption. Express preemption occurs when a statute states directly that it displaces state law. Implied preemption arises either because federal regulation occupies an entire field, or because compliance with both laws is impossible or the state law obstructs the federal purpose.
When none of these conditions is met, state and federal law can coexist. Preemption analysis therefore turns closely on what Congress intended and on how sharply the two rules actually conflict.