Where federal and state law conflict, federal law prevails. The analysis proceeds in recognised categories.
Express preemption. A federal statute states that it displaces state law. The work is in construing the preemption clause’s scope, and courts often read it alongside savings clauses preserving some state authority.
Field preemption. The federal scheme is so pervasive, or the federal interest so dominant, that Congress is inferred to have left no room for state regulation. Immigration, nuclear safety and certain aspects of labour relations are traditional examples.
Conflict preemption. Either compliance with both is impossible, or state law stands as an obstacle to the accomplishment of the federal purposes. Obstacle preemption is the most contested category.
The presumption against preemption applies with particular force in fields traditionally occupied by the states — health, safety, land use, property and family law — though its strength is debated.
Agency action. Regulations may preempt within the scope of the agency’s delegated authority, and an agency’s own view of preemptive effect receives some weight.
Practical application for local government. Before adopting an ordinance in a federally regulated area — telecommunications, banking, transportation, food labelling, firearms in some respects — the preemption analysis should be part of the drafting file rather than the litigation file.