It was produced by Filmation and ran for two seasons 1972-1973, airing a total of twenty-two thirty-minute episodes. This show featured most of the original cast performing the voices for their characters, except for Pavel Chekov (Walter Koenig), who was replaced by Lieutenant Arex. Arex was a member of a tripodal species which had three arms and three legs. Another semi-regular addition to the cast was Lt. M'Ress, a female cat-like alien.
While one would believe that Star Trek could take full use of the freedom that animation provided, budget restraints were still a concern, and the animation quality was poor. However, it did afford the crew a set of much more encompassing alien landscapes than could be provided by a small studio.
Noted science fiction authors such as David Gerrold and Larry Niven contributed scripts for the animated series. In addition to a sequel to the famous episode The Trouble with Tribbles, two notable episodes of the cartoon include Yesteryear (a time-travel episode in which Mr. Spock uses the time gateway from the live-action episode City on the Edge of Forever to travel to his own childhood past) and Niven's The Slaver Weapon (adapted from his own short story "The Soft Weapon"). Larry Niven in particular included some elements his Known Space mythos, such as the Kzinti and the Slavers.
It is of note that the Enterprise ship in this series, while theoretically the same ship on the live action program, had a holodeck just like was introduced on Star Trek The Next Generation.
For a variety of reasons (most likely legal ones), Paramount Pictures does not consider The Animated Series to have 'actually' happened in the Star Trek universe. There have however been occasions where writers and other production crew have sneaked in references to the animated series into the live-action series, to many a Trekkie's delight.
See Also: Episode Listing