Admission to the Union is provided by the New States Clause of the United States Constitution in Article IV, Section 3, Clause 1, which authorizes the United States Congress to admit new states into the Union beyond the thirteen states that already existed when the Constitution came into effect. The Constitution went into effect on June 21, 1788 in the nine states that had ratified it, and the U.S. federal government began operations under it on March 4, 1789, when it was in effect in 11 of the 13 states. Since then, 37 states have been admitted into the Union. Each new state has been admitted on an equal footing with those already in existence.Of the 37 states admitted to the Union by Congress, all but six have been established within existing U.S. organized incorporated territories. A state that was so created might encompass all or part of a territory. When the people of a territory or a region have grown to a sufficient population and have made their desire for statehood known to the federal government, Congress in most cases has passed an enabling act, authorizing the people of that territory or region to frame a proposed state constitution as a step toward admission to the Union. The use of an enabling act has been a common historic practice, but several states were admitted to the Union without one.

In many instances, an enabling act would detail the mechanism by which the territory would be admitted as a state after the ratification of their constitution and the election of state officers. Although the use of such an act is a traditional historic practice, several territories have drafted constitutions for submission to Congress absent an enabling act but were subsequently admitted. The broad outline for the process was established by the Land Ordinance of 1784 and the 1787 Northwest Ordinance, both of which predate the U.S. Constitution.

The Admission to the Union Clause forbids the creation of new states from parts of existing states without the consent of all of the affected states and that of Congress. The primary intent of the caveat was to give the four Eastern States that still had western land claims (Connecticut, Georgia, North Carolina, and Virginia) a veto over whether their western counties could become states. The clause has since served the same function each time that a proposal to partition an existing state or states has arisen.