The President of the United States oath of office was established in Article II of the U.S. Constitution, and is mandatory for a new President before taking office, or for a re-elected President before beginning a new term. The oath is typically administered by the Chief Justice of the Supreme Court of the United States, and is usually taken with the incoming President's hand on a Bible. The oath goes as follows:

I, [insert name], do solemnly swear [or affirm] that I will faithful execute the office of President of the United States, and that I will, to the best of my ability, preserve, protect, and defend the Constitution of the United States.

The moment the oath is taken the recipient is officially President. George Washington set the precedent of adding the line "so help me God," and kissing the Bible upon completion of the oath, however this is not necessary, nor is the use of a Bible. John Tyler, Franklin Pierce and Herbert Hoover was the few President sto take the option of affirming rather than swearing the oath.