On Inauguration Day, the President-elect and Vice President-elect swear in and take office.
"I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States." -- Article II, Section 1, clause 8 - U.S. Constitution
Visit USA.gov's President Barack Obama page for information about Inauguration Day events. You can also learn about inaugural history, download the 57th Presidential Inauguration mobile app, and view a video about the ceremony location.
Did you know that not all of the US Presidents were inaugurated?
Facts, Firsts and Precedents about US Presidential Inaugurations