States' Rights, in United States history, a political doctrine that upholds the power of states in relation to that of the national government. There has been continuous, and at times intense, debate regarding the constitutional extent of federal powers and the rights of the individual states.

In general, states' rights adherents hold that the 10th Amendment to the U.S. Constitution, which refers to the reserved powers of the states, should be interpreted strictly; that is, all powers and functions not specifically delegated to the federal government are reserved to the states. This interpretation would restrict the assumption of implied powers by the federal government under Article I, Section 8. The balance between state and national power, however, usually has been determined not so much by the language of the Constitution as by the strength of political forces.

Up to the Civil War, the states' rights debate centered around the theory of state sovereignty. Because the states were sovereign under the Articles of Confederation (1781–89), some leaders claimed that the states remained sovereign under the Constitution, retaining the power to secede from the Union or to nullify acts of the U.S. Congress. The first important expression of this theory was the Kentucky and Virginia Resolutions of 1798–99. Advocates of state sovereignty asserted the rights of nullification and secession in a series of increasingly bitter political debates during the first half of the 19th century. ( The question of state sovereignty was finally resolved by force of arms, with the defeat of the secessionist South in the Civil War.

In recent times, states' rights adherents have opposed the concentration of power in the national government and the exercise of federal authority in matters considered by them to be the exclusive concern of the states, such as education, racial relations, and welfare expenditures. The historical trend, however, has been toward the gradual broadening of the powers of the federal government.