Interposition is a claimed right of a U.S. state to oppose actions of the federal government that the state deems unconstitutional. Under the theory of interposition, a state assumes the right to "interpose" itself between the federal government and the people of the state by taking action to prevent the federal government from enforcing laws that the state considers unconstitutional.

The theory of interposition is grounded in the text of the Tenth Amendment, which states: "The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people."

In Cooper v. Aaron, 358 U.S. 1 (1958), the Supreme Court of the United States rejected interposition explicitly. The Supreme Court and the lower federal courts have consistently held that the power to declare federal laws unconstitutional lies with the federal judiciary, not with the states. The courts have held that interposition is not a valid constitutional doctrine when invoked to block enforcement of federal law.

Interposition is closely related to the theory of nullification, which holds that the states have the right to nullify federal laws that are deemed unconstitutional and to prevent enforcement of such laws within their borders.

Though interposition and nullification are similar, there are some differences. Nullification is an act of an individual state, while interposition was conceived as an action that would be undertaken by states acting jointly. Nullification is a declaration by a state that a federal law is unconstitutional accompanied by a declaration that the law is void and may not be enforced in the state. Interposition also involves a declaration by a state that a federal law is unconstitutional, but interposition as originally conceived does not result in a declaration by the state that the federal law may not be enforced in the state. Rather, the law would still be enforced. Thus, interposition may be seen as more moderate than nullification.

There are various actions that a state might take to "interpose" itself once it has determined that a federal law is unconstitutional. These actions include communicating with other states about the unconstitutional law, attempting to enlist the support of other states, petitioning Congress to repeal the law, introducing Constitutional amendments in Congress, or calling a constitutional convention.

Interposition and nullification often are discussed together, and many of the same principles apply to both theories. In practice, the terms nullification and interposition often have been used indistinguishably. John C. Calhoun indicated that these terms were interchangeable, stating: "This right of interposition, thus solemnly asserted by the State of Virginia, be it called what it may — State-right, veto, nullification, or by any other name — I conceive to be the fundamental principle of our system." During the fight over desegregation of the schools in the south in the 1950s, a number of southern states tried to preserve their segregated schools by passing so-called "Acts of Interposition" that actually would have had the effect of nullification, if they had been valid. These acts were struck down by the courts, whether labelled acts of interposition or nullification.