The U.S. Senate is the upper house of the Congress of the United States. It consists of two senators from every state (unlike the House of Representatives, whose members are apportioned according to the population of each state). The Senate has 100 members. As of January 2015 (the 114th Congress), the Senate included 54 Republicans, 44 Democrats, and 2 independents who caucus with the Democrats.

Senators serve 6-year terms, with a third of the terms expiring every 2 years. Originally, the Constitution provided for their selection by the state legislatures, in contrast to the popular election of members of the House. The 17th Amendment, adopted in 1913, required that senators be elected by the people. Senators must be 30 years of age, citizens of the United States for at least 9 years, and residents of the states they represent.

With the exception of four years (1947–48 and 1953–54), the Democrats controlled the Senate from 1933 through 1980. Since then, power has changed hands more often. Republicans controlled the chamber in 1981–86, 1995–2006 (except May 2001–December 2002), and 2015–    . Democrats controlled it in 1987–94, May 2001–December 2002, and 2007–14. Unfortunately, the uncertainty of power in the more recent period (together with other factors) appears to have contributed to a growing dysfunction. When there was little doubt about continued party control, at least some politicians were willing to cooperate across party lines to achieve common goals. As the possibility of gaining power in the next election became more real, the minority party lost its incentive to cooperate in ways that might accrue to the advantage of the majority party. Obstruction for its own sake became a goal. Thus, recent years have been marked by political maneuvering and legislative stalemate.

Every act of Congress must be approved by both the House of Representatives and the Senate. In addition, the U.S. Constitution gives the Senate certain unique powers. It has sole authority to ratify, by a two-thirds vote, treaties proposed by the president of the United States. The Senate also has the sole power to accept or reject—by majority vote—certain presidential appointments. These include federal judgeships (including the Supreme Court), ambassadorships, cabinet posts, and high-level positions in the executive branch of the federal government.

Senate rejection of treaties and presidential nominations is rare but not unprecedented. After World War I it refused to ratify the Treaty of Versailles (see Paris Peace Conference) as signed by President Woodrow Wilson. In 1999 it rejected the Comprehensive Test Ban Treaty. It rejected two of President Richard M. Nixon's nominations of Supreme Court justices in 1970 and one of President Ronald Reagan's in 1987. While the House of Representatives has sole authority to impeach public officials, the resulting trial must be conducted in the Senate, with a two-thirds vote necessary for conviction (see Impeachment). Between 1797 and 2010, the Senate conducted formal impeachment proceedings 19 times. These involved 2 presidents, 1 cabinet officer, 1 senator, 1 Supreme Court justice, and 14 federal judges. The proceedings resulted in 7 acquittals and 8 convictions. In addition, 3 judges resigned before the Senate took final action, and the senator was simply expelled from the Senate. The most famous impeachment cases were those of President Andrew Johnson and President Bill Clinton. Johnson was acquitted (1868) by a margin of one vote, and Clinton was comfortably acquitted (1999).

The vice-president of the United States serves as president of the Senate. The Senate chooses its other officers and leaders, including a president pro tempore, who acts as Senate president in the vice-president's absence. The practical powers of these two titular leaders are not wide-ranging. The vice-president rarely presides over routine day-to-day debate in the Senate, being present only at times of important votes or parliamentary decisions. The vice-president has no vote unless the members of the Senate cast a tie vote. A notable modern instance of Senate action by a vice-president was Al Gore's Aug. 6, 1993, tie-breaking vote on President Clinton's budget.

The position of president pro tempore of the Senate is largely honorific. It has traditionally been filled by the most senior senator of the majority party—the one with the longest continuous service in the chamber. Although there is no mention of political parties in the Constitution, the Senate since its earliest days has organized and chosen its leaders along political party lines. This organization is based on a two-party division into majority and minority.

The most important figure in the Senate is the majority leader, elected by the members of the majority party. The majority leader is aided by an assistant majority leader, also known as the majority whip. The majority leader directs Senate floor activity and helps coordinate the debate and timing of legislation. The primary political resources of the leader are prestige, parliamentary knowledge, and powers of persuasion. The majority leader also presides over the collective caucus, or conference, of members of the majority party. He or she plays a major role in allocating assignments to committees for majority senators. The views and interests of the minority party in the Senate are represented by similar officers.

The business of the Senate is conducted through a highly structured committee system. In 2015 there were 16 standing committees (which consider and report all legislation), 2 investigative select committees (on ethics and intelligence), 1 other permanent committee (on Indian affairs), and 1 special committee (on aging). The committees had nearly 70 subcommittees, each with the power to hold hearings and conduct investigations. There were also 4 joint committees, which include both Senate and House members. Individual senators served, on an average, on 3 committees and 8 subcommittees. Major standing committees include Appropriations; Armed Services; Budget; Commerce, Science, and Transportation; Energy and Natural Resources; Finance; Foreign Relations; Health, Education, Labor, and Pensions; Homeland Security and Government Affairs; and Judiciary.

The chairpersons of committees, who have considerable prestige and influence, are nominally selected by the senators of the majority party. In practice, the determining factor is seniority—with the qualification, adopted by the Republicans in 1995 and effective beginning in 1997, that their chairs be limited to six years. In 1977 the Democrats switched to secret balloting in selecting committee heads, but the seniority custom was not breached in practice.

The 2000 elections resulted in a unique series of events. The Senate entered the new century with an equal number of Democrats and Republicans. This was the first time an even split had occurred since 1880. The Democrats sought a power-sharing arrangement whereby they would share committee chairmanships with the previously dominant Republicans and take a hand in scheduling bills and hiring staff. The Republicans successfully argued that they were the majority because Dick Cheney—who was the vice-president and, therefore, president of the Senate—was from their party and could vote to break ties. Leadership positions and committee chairs, therefore, all went to Republicans. The two sides agreed that Republicans and Democrats would have an equal number of seats on committees.

In May 2001, however, Sen. James Jeffords of Vermont left the Republican party, became an independent, and caucused with the Democrats. Power thus shifted in the middle of the congressional session. This was the first time that a member's change of affiliation had caused control of the Senate to change hands. Democrats assumed all Senate leadership positions and committee chairs. The Republicans regained control in the next election.

The Senate determines its own rules. Perhaps the most distinctive feature of Senate procedure is its tradition of extended and open debate—that is, any senator can continue debating a bill or an issue indefinitely, unless two-thirds of the senators present and voting adopt a motion of cloture. The use of dilatory tactics to stymie or delay action is known as the filibuster. During the 1950s and 1960s, Southern Democratic senators frequently employed filibusters to delay or quash civil rights measures. A team of senators talked for 57 days in an attempt to stop the Civil Rights Act of 1957. In 1975 the Senate altered its standing Rule XXII to permit 60 senators to end a filibuster unless the debate concerned Senate rules (in which case the two-thirds rule remained in effect). Budget resolutions could be passed with a simple majority. At the same time, the nature of the filibuster was altered. No longer would a senator need to speak continuously, disrupting all Senate business in order to stop action on one bill. Instead, he or she would merely register an intention to filibuster a bill and allow the Senate to continue working on other issues. Action on the filibustered bill would require a successful cloture vote. Thus, the threshold for ending a filibuster was lowered, but the process of conducting a filibuster was greatly eased.

Use of the filibuster expanded in the 1990s and 2000s. During the administration of President Barack Obama (2009–17), virtually every bill in the Senate was filibustered. Nothing could pass the Senate without 60 votes. In general, increasing use of the filibuster by the minority to frustrate majority initiatives would give rise to calls to end the practice for good. One such confrontation, over the nomination of judges in 2005, ended in a bipartisan deal to confirm a certain number of judges and to allow the filibuster rule to continue unchanged. Another confrontation, in 2013, ended with the majority Democrats unilaterally "reinterpreting" an existing Senate rule. Henceforth, cloture on executive and judicial nominations (other than Supreme Court justices) would require only a simple majority. (The threshold for legislation remained at 60.) The Republicans retained the new interpretation when they gained control of the Senate in 2015.

Because of its smaller size and unique powers, the Senate is commonly regarded as the more prestigious and influential of the two houses of Congress, although House members would disagree, pointing to their primacy in the Constitution and their own unique role. Senators like to refer to the Senate as "the greatest deliberative body in the world."