As of 2015, there are 100 members of the U.S. Senate. Since the U.S. Constitution sets the number of senators at two per state, this number has held steady since the admission of Hawaii as the 50th state in 1959.
The age, citizenship and state of residency of an individual determine their eligibility to serve as a U.S. Senator. In order to serve as a senator, an individual cannot be less than 35 years of age, must live, at the time of their election, in the state voters elected them to represent and they must have been a U.S. citizen for at least nine years. A senator serves a term of six years and approximately 1/3 of the Senate is up for election every two years.