"Tomorrow Is Yesterday" is the nineteenth episode of the first season of the American science fiction television series Star Trek.

When the Enterprise is labeled a UFO, a U.S. Air Force F-104 interceptor, piloted by Captain John Christopher, is scrambled to identify the craft.

When Science Officer Spock later discovers that the pilot's as-yet-unborn son will play an important role in a future mission to Saturn, Kirk realizes he must return Christopher to Earth after all.

[2] Associate producer Robert Justman devised the original idea for the story, and it was handed to Dorothy Fontana to create a teleplay.

The second issue of IDW Publishing's comic book series Assignment: Earth (a continuation of the episode of the same name, drawing on the episode's status as a failed backdoor pilot for a spin-off television series) shows the protagonists, Gary Seven and his assistant Roberta Lincoln, becoming peripherally involved in the events of "Tomorrow is Yesterday", acting on their own to prevent the Enterprise's presence from affecting history.

The "slingshot maneuver" was employed a third time by the crew in the motion picture Star Trek IV: The Voyage Home, this time to travel back to the year 1986, and to return the Klingon Bird of Prey, renamed Bounty, which the command crew had captured at the end of Star Trek III: The Search for Spock, to 2286.

Dr. Agnes Jurati notes in "Penance", the twelfth episode of Star Trek: Picard, that "Kirk's Enterprise" used the method multiple times, and proposed doing the same, in order to restore their timeline.

Responding to the objection that Kirk was able to succeed only because he had Spock to perform the calculations, Jurarti struck a deal for the services of the captive Borg Queen who was just as eager to set the timeline aright.

In 2009, Tor.com rated it 5 out 6, noting that it depicted a sort of "hope and optimism and excitement about the new frontier" at a time before the first lunar landing mission.