Preemption is a legal doctrine that allows a higher level of government to limit or even eliminate the power of a lower level of government to regulate a particular issue. When two authorities come into conflict, the higher authority of law will displace the law of the lower authority of law. There are two types of preemption: federal preemption and state preemption.

• Federal Preemption: This occurs when a U.S. state law conflicts with federal law, and the federal law invalidates the state law. Federal preemption can be expressed or implied. Express preemption occurs when a federal statute explicitly confirms Congresss intention to preempt state law, while implied preemption can occur in two ways: field preemption or conflict preemption.

• State Preemption: This occurs when a state law conflicts with a local law, and the state law prevails. State preemption can be outright, express, or implied. Outright conflict occurs when an ordinance directly opposes a state law, express preemption occurs when the state law directly opposes a local power, and implied preemption occurs when a local ordinance prohibits an act permitted by the state legislature, when a local ordinance permits an act prohibited by the state legislature, or when there is clear legislative intent that the "field" is preempted by state law.

Preemption can have sweeping consequences for local policymaking, and it is a complicated policy question that is resolved in the halls of Congress.