(written from a Production point of view)
A list of episodes where we see an alternate timeline. For a list of episodes where we see a parallel universe see Parallel universe episodes. For a list of episodes that feature actual travel through time see Time travel episodes.
Star Trek: The Original Series Edit
|"The City on the Edge of Forever"||1930-2267||Edith Keeler's death averted by McCoy||Nazi Germany lasts beyond 1945, United Federation of Planets does not exist|
Star Trek: The Animated Series Edit
|"Yesteryear"||2269, 2237||Death of Spock as a child||Amanda Grayson has died, Thelin became USS Enterprise's first officer|
Star Trek: The Next Generation Edit
|"Yesterday's Enterprise"||2366||USS Enterprise-C went through a temporal rift in 2344||Federation embroiled in long war with Klingons|
"Yesterday's Enterprise"'s changes in detail:
By 2366, 22 years of war have raged between Klingons and the Federation, with the death toll in the billions. At that point, the war was going badly for the Federation, and a surrender seemed the most likely outcome.
The USS Enterprise, while retaining her exterior, has been fitted as a battleship, with less luxury than the prime timeline USS Enterprise.
The Enterprise has been fighting the Klingons for the past 4 years, and many, if not all, of the missions the 'prime' Enterprise undertook have not been undertaken by this Enterprise.
The Enterprise had no children on board as a result of the war.
Wesley Crusher was a full ensign in 2366, which implies either an earlier acceptance into Starfleet Academy or, as seems more likely in this timeline, that he was drafted into the military.
Star Trek: Deep Space Nine Edit
|"Past Tense, Part I"||2024, 2048, 2371||Gabriel Bell dies||Federation does not exist, Romulans encroach on Alpha Centauri|
|"Past Tense, Part II"||2024, 2048, 2371||Gabriel Bell dies||Federation does not exist, Romulans encroach on Alpha Centauri|
|"Accession"||9174 (Bajoran year) in the 22nd century (Earth calendar)||Akorem Laan disappears from history, entering the Bajoran wormhole||Akorem doesn't complete his poem The Call of the Prophets|
Star Trek: Voyager Edit
|"Non Sequitur"||2372||Timestream||Harry Kim did not join USS Voyager, Daniel Byrd joined instead|
|"Year of Hell"||2374||Krenim weapon ship||Numerous incursions|
|"Year of Hell, Part II"||2374||Krenim weapon ship||Numerous incursions|
Star Trek: Enterprise Edit
|"Twilight"||2153-2165||Interspatial parasites||Archer removed as Captain of Enterprise NX-01, Xindi destruction of Earth and Human colonies|