Presiding officer of the United States Senate

The presiding officer of the United States Senate is charged with maintaining order and decorum, recognizing members to speak, and interpreting the Senate's rules, practices, and precedents. The role of presiding officer may be performed by one of three officials: the vice president of the United States; an elected United States senator (president pro tempore, who fills in the role in the event of an incapacity or vacancy); or, under certain circumstances, the chief justice of the United States. If the Senate convenes with neither the vice president nor a president pro tempore (as with William R. King's absence in 1853), the Secretary or Assistant Secretary supervises election of a president pro tempore. Outside the constitutionally mandated roles, the appointment of a person to do the job of presiding over the Senate as a body is governed by Rule I of the Standing Rules.

The United States Constitution establishes the vice president as president of the Senate, with the authority to cast a tie-breaking vote. Early vice presidents took an active role in regularly presiding over proceedings of the body, with the president pro tempore only being called on during the vice president's absence. During the 20th century, the role of the vice president evolved into more of an executive branch position. Now, the vice president is usually seen as an integral part of a president's administration and presides over the Senate only on ceremonial occasions or when a tie-breaking vote may be needed.

The Constitution also provides for the selection of a president pro tempore of the Senate, to preside when the vice president is absent from the body (as the meaning of pro tempore, literally "for the time being"). The Constitution does not specify who can serve in this position, but the Senate has always elected one of its current members. By tradition, the position is generally given to the most senior senator of the majority party. In actual practice in the modern Senate, the president pro tempore also does not often serve in the role (though it is their constitutional right to do so). Instead, as governed by Rule I, they frequently designate a junior senator to preside.

When the Senate hears an impeachment trial of the incumbent president of the United States, by the procedure established in the Constitution, the chief justice presides.