The Senate is composed of 100 Senators, 2 for each state. 

Until the ratification of the 17th Amendment in 1913, Senators were chosen by state legislatures, not by popular vote. Since then, they have been elected to six-year terms by the people of each state. Senator's terms are staggered so that about one-third of the Senate is up for reelection every two years. 

Senators must be 30 years of age, U.S. citizens for at least nine years, and residents of the state they represent.

The Vice President of the United States serves as President of the Senate and may cast the decisive vote in the event of a tie in the Senate.

The Senate has the sole power to confirm those of the President's appointments that require consent, and to ratify treaties. There are, however, two exceptions to this rule: the House must also approve appointments to the Vice Presidency and any treaty that involves foreign trade. The Senate also tries impeachment cases for federal officials referred to it by the House.