U.S. senators are elected to a single six-year term. They may be elected to any number of successive terms if they should choose to run for re-election.
Each state elects two senators, each of whom holds one vote in the senate chamber. Every two years, approximately one-third of the senate comes up for re-election. Article I Section III of the United States Constitution requires that a senator must be at least 30 years old and live in the state they represent at the time of their election. They must have U.S. citizenship for at least nine years. Prior to 1913, senators were elected by the legislatures of the various states, but the passage of the 17th Amendment altered the process so that senators would instead be directly elected by state residents.