Star Trek is a science fiction television series created by Gene Roddenberry which aired from September 8, 1966 through June 3, 1969. 80 episodes were produced, 79 of which were aired. Although cancelled after a relatively short run, the program was placed in syndication, where it spawned a strong fan following and, later, achieved iconic status as an American - and eventually worldwide - television phenomenon.

Set in the 23rd century, Star Trek follows the adventures of the starship Enterprise and her crew, led by Captain James T. Kirk (William Shatner) and his First Officer Mr. Spock (Leonard Nimoy). Bill Shatner's legendary introduction to the show stated the starship's purpose and encapsulates the enduring draw for the fans then and now:

The success of the program was followed by five additional television series and eleven theatrical movies. The Guinness Book of Records lists it as having the largest number of spinoffs. Though the title of the original program was simply Star Trek, it has acquired the retronym Star Trek: The Original Series (sometimes shortened to ST:TOS or TOS), in order to distinguish this first series from the sequels which followed (all of which comprise the Star Trek universe or franchise). It was also sometimes known as Classic Trek.

When Star Trek debuted on NBC in 1966, it was not successful; ratings were low and advertising revenue was lackluster. During the show's second season, the threat of cancellation loomed. The show's devoted fanbase conducted an unprecedented letter-writing campaign, petitioning NBC to keep the show on the air. The fans succeeded in gaining a third season, however NBC moved the show to a certain Friday Night Death Slot, and ratings remained poor. The series was cancelled at the end of its third season. The show became successful in reruns, and thus the Star Trek saga was born.