As of December 2014, only 27 constitutional amendments exist, though many different versions of a 28th Amendment have been suggested. At different times, overwhelming majorities in several states have passed their own versions of the 28th Amendment, but none have been ratified.
In all, 33 amendments to the U.S. Constitution have been proposed, but five have failed to pass the ratification process as outlined in Article V of the Constitution. The ratification process requires that two-thirds of both the House of Representatives and the Senate pass the proposed amendment, after which the amendment is put to the states to ratify. If an amendment is not approved by three-fourths of the states, it is not considered an operative part of the U.S. Constitution.
It should be observed that failure to approve or take action on a proposed amendment does not preclude a state from abiding by said amendment. The last constitutional amendment to be ratified was submitted to the states for ratification on Sept. 25, 1789, and became an operative part of the Constitution on May 7, 1992, requiring a total of 202 years, seven months and 12 days. Once ratified, the 27th Amendment delayed changes to congressional salary, preventing any changes from taking effect until after the next election cycle.