In the fictional Star Trek universe, the United Federation of Planets (UFP) is the interstellar government that sent Captain Kirk, Mr. Spock, and the crew of the starship Enterprise on its mission of peaceful exploration. Commonly referred to as "the Federation", it was introduced in the television show Star Trek (1966-1969). The survival, success, and growth of the Federation and its principles of freedom have become some of the Star Trek franchise's central themes. The Federation is an organization of numerous planetary sovereignties, and although viewers are never told about the internal workings of the government, many episodes refer to the rules and laws that the Federation imposes on the characters and their adventures.