(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
|"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
|"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
|"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:
In 2366, 22 years of war have been raging between Klingons and the Federation, with the death toll in the billions. At that point, the war went badly for the Federation, and a surrender would seem the most likely outcome.
The USS Enterprise, while retaining her exterior, has been fitted as a battleship, with less luxury as the prime timeline USS Enterprise.
The Enterprise has been fighting the Klingons in the past 4 years, and many, if not all, of the missions the 'prime' Enterprise undertook were never undertaken by this Enterprise.
The Enterprise had no children on board as a result of the war.
Wesley Crusher was full ensign in 2366 in this timeline, which implies either an earlier acceptance into Starfleet Academy or, as seems more likely in this timeframe, that he has been drafted into the military.
Star Trek: Deep Space Nine
|"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|
Star Trek: Voyager
|"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
|"Twilight"||2153-2165||Interspatial parasites||Archer removed as Captain of Enterprise NX-01, Xindi destruction of Earth and human colonies|