"All Our Yesterdays" is a third season (and the ) episode of the science fiction television series, Star Trek, first broadcast March 14, 1969, and repeated on August 5, 1969. It is episode #78, production #78, written by Jean Lisette Aroeste, and directed by Marvin Chomsky. It guest-stars Mariette Hartley as Zarabeth. The title comes from a well-known soliloquy given by Macbeth in Act V Scene V.

In this episode, Kirk, Spock and McCoy are trapped in two parts of another planet's past—a world threatened with destruction when its sun goes supernova.

The USS Enterprise arrives at the planet Sarpeidon, whose star is soon to go supernova. Surprised to find the surface devoid of humanoid life, Kirk, Dr. McCoy and Spock beam down to investigate. They encounter one last remaining resident, a librarian named Mr. Atoz who can somehow appear anywhere at will and scolds them for being very late. Aware of the imminent destruction, Mr. Atoz tells the perplexed landing party that he will soon escape to rejoin his own family. Mr. Atoz shows them the Atavachron, a machine with a time portal that connects to Sarpeidon's past documented on the library's discs. Suddenly, they hear a woman scream. Kirk instinctively runs through the portal and McCoy and Spock follow. Mr. Atoz tries to warn them all that they were not "prepared".

Kirk instantly finds himself alone and in the midst of a duel in a period similar to England in Earth's 17th century while McCoy and Spock are 5,000 years back in Sarpeidon's ice age. They cannot locate the portal's entrance to return but the three are able to speak to each other. Spock correctly surmises that the Sarpeidons have all escaped to their planet's past.

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