In the United States, the office of the president of the Senate is held by the nation’s vice president. Despite what his title may suggest, the office actually has fairly few duties in practice. The main responsibilities include casting votes when ties have occurred and occasionally presiding over the Senate in a ceremonial capacity. When the Senate's president is not present — as regularly happens — an elected figure called the president pro tempore presides.
Perhaps the most significant power awarded to the president of the Senate is that of casting deciding votes. This prerogative is exercised only when the results of a Senate vote on a given issue are equally split, 50-50. In this case, the officeholder, if present, may opt to vote, deciding the issue. If he or she is absent in a tie situation, the motion fails. It is only when the Senate has arrived at a tie that this person is allowed to cast a vote.
At times, the Senate’s president also presides over ceremonial events. He or she may, for instance, be present to swear in new senators. In addition, he or she may preside on those occasions when the Senate and the House of Representatives meet together for shared discussion or to listen to an address given by the US president. These meetings are known as joint sessions.
As the individual serving as the president of the Senate is simultaneously serving as the United States vice president, he or she is frequently detained by executive duties and is, therefore, unable to preside over a large proportion of Senate activities. Consequently, the president pro tempore frequently takes over the day-to-day duties of leading Senate business. This includes pronouncing vote results and inviting senators to address the chamber. Though the president pro tempore must win his seat through a Senate election, in most cases, he or she is the longest-serving member of the Senate majority party.
Often, however, the president pro tempore passes on these daily duties to junior senators from the majority party, who preside over the Senate for short periods. This experience enables them to gain a valuable working knowledge of Senate policies and processes.