The original series, created by Gene Roddenberry, lasted three seasons (1966-1969), although it was almost cancelled at the end of its second season and was reportedly saved only through a letter-writing campaign. But the show, with 79 episodes, became a hit in syndicated re-runs.
While many elements of the Enterprise were present from the start, the series only slowly built its universe, as required for its episodic stories. The first season established the United Federation of Planets, Starfleet, Starfleet Command, Starbases, and the idea that there were twelve ships like the Enterprise in service. Chekov was added for the second season, which expanded on Spock’s heritage. While episodic, a few episodes did make reference to past adventures. Exactly when the show was set remained vague throughout, with contradictory references to Earth’s past, because Gene Roddenberry preferred to leave the setting vague.
On a number of occasions, the series used its outer-space setting to reflect issues of the time, especially the Cold War. The series also frequently featured humor, an element later series lacked, and episodes often ended with a joke or with laughter, cementing the core characters’ camaraderie.
After four seasons off the air, the show returned in 1973 as a half-hour, animated series, produced by Filmation. All of the principal characters returned, voiced by their original actors, except Chekov. The series made relatively mild changes to the Enterprise’s technology and crew, but it remained in tone with the original series — with the exception of a prohibition against killing crew members, which occurred only in a single animated episode. The series also used the medium of animation to produce more elaborate alien landscapes and non-human-looking characters than were present on the original, live-action series.
Animated shows at the time rarely had long lives. The first season won an Emmy Award, which was reportedly the only reason it was renewed for an abridged second season. In total, 22 episodes were produced.
Although the animated series was regarded as non-canonical for years afterward, a view advanced by Roddenberry, it introduced several elements that remained as part of the Star Trek canon and were referenced by later productions. The show’s voice actors and writers, however, largely regarded it as effectively constituting a fourth season of the original show.