The United States Constitution has been amended 27 times since it was signed in 1787.

The procedures for amending the US Constitution are outlined in Article V:

The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution, or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case, shall be valid to all Intents and Purposes, as Part of this Constitution, when ratified by the Legislatures of three fourths of the several States, or by Conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress; Provided that no Amendment which may be made prior to the Year One thousand eight hundred and eight shall in any Manner affect the first and fourth Clauses in the Ninth Section of the first Article; and that no State, without its Consent, shall be deprived of its equal Suffrage in the Senate.

All of the amendments that have been ratified have been through the first path outlined in Article V: they have first been passed by a 2/3 vote of the House and the Senate, and then have been ratified by 3/4 of the states. Six amendments have been approved by Congress, but have not been ratified by the required number of states. Four of these amendments are still pending.

The last time the Constitution was amended was in 1992, when the 27th Amendment, which was passed by the 1st Congress in 1789, was finally ratified by three fourths of the states.