According to the doctrine of preemption, if a federal law preempts a state law, then the state law is declared invalid. This Doctrine is based on the Supremacy Clause of the U.S. Constitution. The Supremacy clause has come to mean that national exercise of power in accordance with the constitution must prevail over conflicting state exercise of power.


  • Explicit conflict – if a state law obviously conflicts with federal law than the state law is automatically struck down.
  • Field Preemption – Even if state law does not explicitly conflict with federal law, it can still be struck down if congress has legitimately occupied the field with federal legislation. Field Preemption usually involves balancing of important state and federal interests. Often times Congress fails to make its purpose clear and the court has to make inferences based on the objectives of the federal law and the impact of related state law.


  • Commerce Clause of the U.S. Constitution has resulted in field preemption of state labor laws which has made state and local laws that interfere with federal environmental laws invalid.
  • Federal Power Act preempts state regulations imposing minimum flow rates on rivers for hydroelectric power.

State Level

Preemption also exists at the state level. State exercise of power will preempt local laws. Preemption at the state level varies based on the state constitution but follows the process of field preemption. When the state legislature enacts legislation and the intent in doing so is to occupy that field than local municipalities will be preempted from enacting their own legislation within that field.

For example, if state legislature enacts gun control laws and their intent in doing so is to occupy the field of gun control, than local laws governing gun control will be preempted and invalid.


The doctrine of preemption is extremely powerful. When involved with a lawsuit that implicates certain state and local laws it is important to first check the possible applicability of preemption. The doctrine of preemption can apply at times even when there appears to be no explicit conflict. Field Preemption is a powerful tool that can be used to challenge state and local laws that appear to be less favorable than existing federal laws within the same field.