The President pro tempore of the Senate is the second-highest official in the United States Senate, after the Vice President, who is the President ex officio of the Senate. Typically, the President pro tempore is the senior senator of the majority party, and the highest ranking member of the Senate as a result. This position is primarily ceremonial, and it comes with numerous responsibilities and rather limited powers. However, it is considered a great honor to serve as the President pro tempore of the Senate, not least because being in this position puts a Senator third in the line of Presidential succession.

This position was first outlined in 1789 in the Constitution. When the Vice President is not available to preside over the Senate, the President pro tempore has the power to preside, governing proceedings and making rulings on points of procedure. It is not uncommon for the President pro tempore to allow a junior senator to preside, giving the less experienced senator a chance to learn more about how the Senate works. The President pro tempore is entitled to a slightly larger staff than other Senators, thanks to the increased responsibility associated with the position.

The Senate officially elects the President pro tempore, although by convention the position is awarded to the senior member of the majority party and the election is typically unanimous. In the event that the party majority changes and the President pro tempore remains in office, he or she becomes the Emeritus President pro tempore as a mark of respect when the position is assumed by the senior member of the new majority party.

It is also possible to see an Acting President pro tempore, an official who is appointed to take over for an incapacitated or disabled President pro tempore. Acting Presidents are not in line for the office of the President, and will be passed over in the event of a situation where the President and Vice President are incapacitated. As a gesture of courtesy, former Presidents and Vice Presidents are considered Deputy Presidents pro tempore, although it is extremely unusual for Presidents and Vice Presidents to serve in the Senate after their terms are over.

A number of ceremonial duties accompany this position, and the President pro tempore is usually expected to preside over major Senator proceedings. Due to his or her senior rank, the President pro tempore also usually has a number of committee obligations and other responsibilities. In recognition for the additional work which accompanies the position, the President pro tempore is compensated more than other Senators. As of 2008, the President pro tempore of the Senate was earning almost $200,000 US annually.