Nullification, in United States constitutional history, is a legal theory that a state has the right to nullify, or invalidate, any federal laws that they deem unconstitutional with respect to the United States Constitution (as opposed to the state's own constitution). There are similar theories that any officer, jury, or individual may do the same. The theory of state nullification has never been legally upheld by federal courts, although jury nullification has.

The theory of nullification is based on a view that the states formed the Union by an agreement (or "compact") among the states, and that as creators of the federal government, the states have the final authority to determine the limits of the power of that government. Under this, the compact theory, the states and not the federal courts are the ultimate interpreters of the extent of the federal government's power. Under this theory, the states therefore may reject, or nullify, federal laws that the states believe are beyond the federal government's constitutional powers. The related idea of interposition is a theory that a state has the right and the duty to "interpose" itself when the federal government enacts laws that the state believes to be unconstitutional. Thomas Jefferson and James Madison set forth the theories of nullification and interposition in the Kentucky and Virginia Resolutions in 1798.

Courts at the state and federal level, including the U.S. Supreme Court, repeatedly have rejected the theory of nullification. The courts have decided that under the Supremacy Clause of the Constitution, federal law is superior to state law, and that under Article III of the Constitution, the federal judiciary has the final power to interpret the Constitution. Therefore, the power to make final decisions about the constitutionality of federal laws lies with the federal courts, not the states, and the states do not have the power to nullify federal laws.

Between 1798 and the beginning of the Civil War in 1861, several states threatened or attempted nullification of various federal laws. None of these efforts were legally upheld. The Kentucky and Virginia Resolutions were rejected by the other states. The Supreme Court rejected nullification attempts in a series of decisions in the 19th century, including Ableman v. Booth, which rejected Wisconsin's attempt to nullify the Fugitive Slave Act. The Civil War ended most nullification efforts.

In the 1950s, southern states attempted to use nullification and interposition to prevent integration of their schools. These attempts failed when the Supreme Court again rejected nullification in Cooper v. Aaron, explicitly holding that the states may not nullify federal law.

The Constitution does not contain any clause expressly providing that the states have the power to declare federal laws unconstitutional.

Supporters of nullification have argued that the states' power of nullification is inherent in the nature of the federal system. They have argued that before the Constitution was ratified, the states essentially were separate nations. Under this theory, the Constitution is a contract, or "compact", among the states by which the states delegated certain powers to the federal government while reserving all other powers to themselves. The states, as parties to the compact, retained the inherent right to judge compliance with the compact. According to supporters of nullification, if the states determine that the federal government has exceeded its delegated powers, the states may declare federal laws unconstitutional. Nullification supporters argue that the power to declare federal laws unconstitutional not only is inherent in the concept of state sovereignty but also is one of the powers reserved to the states by the Tenth Amendment.

This view of the Constitution has been rejected by the federal courts, which consistently have held that under the Constitution, the states do not have the power to nullify federal laws. The courts have rejected the compact theory, finding that the Constitution was not a contract among the states. Rather, the Constitution was established directly by the people, as stated in the preamble: "We the people of the United States ..." The people made the federal government superior to the states in certain ways. Under the Supremacy Clause of Article VI, the Constitution and federal laws made in pursuance thereof are "the supreme law of the land ... anything in the constitution or laws of any state to the contrary notwithstanding." The courts have held that federal laws are therefore superior to state laws and cannot be negated by the states. Federal laws are valid and are controlling, so long as those laws were adopted in pursuance of—that is, consistent with—the Constitution. Determining whether a federal law is consistent with the Constitution requires interpretation of the law, which is inherently a judicial function. The federal judicial power granted by Article III of the Constitution gives the federal courts authority over all cases "arising under this Constitution [or] the laws of the United States". The federal courts, therefore, have been given the power to determine whether federal laws are consistent with the Constitution, with the Supreme Court having final authority.