Article IV: The States

Article IV has four sections. Section 1 requires each state to give full faith and credit to the public acts, records, and court judgments of every other state. Section 2 guarantees that the citizens of each state are entitled to all privileges and immunities of citizens in the several states, which bars states from discriminating against out-of-state citizens in fundamental matters, and requires states to return people charged with crimes who flee to another state. It also contained the Fugitive Slave Clause, made obsolete by the Thirteenth Amendment in 1865.

Section 3 lets Congress admit new states, provided no state is formed from another state's territory or by joining states without their legislatures' consent, and gives Congress power to make all needful rules and regulations respecting the territory and other property of the United States. Thirty-seven states have been admitted since the original thirteen, most recently Hawaii in 1959. The Property Clause underlies federal control of national parks, forests, and other public lands, which cover more than a quarter of the nation's land, and Congress's authority over territories such as Puerto Rico and Guam.

Section 4 guarantees every state a republican form of government and protection against invasion and, on request, against domestic violence. Article IV remains central to current debates, including statehood proposals for the District of Columbia and Puerto Rico, the status of the territories under the Insular Cases, interstate conflicts over abortion and shield laws, and disputes between Western states and the federal government over public lands. The Supreme Court in Saenz v. Roe in 1999 grounded part of the right to travel between states in related privileges and immunities protections.