Because each game of the Fallout series was created by a different development team and the plot and dialogues were created by mostly different people each time around, there are numerous inconsistencies between them and the canonicity of each game is a point of contention between various Fallout fans. For example, in case of inconsistency between games, some fans might consider newer entries in the series to override the older ones, while others might consider the original lore to still be "true" and inconsistencies to be mistakes on the part of the later titles' developers. Even various developers of one game might disagree on what holds true in the games' setting: for example, Tim Cain and Chris Taylor have different views on the origins of ghouls.
Since the acquisition of the Fallout franchise by Bethesda Softworks and their development of Fallout 3, it is Bethesda that defines the official canon. However, many Fallout fans prefer the Fallout canon as it was defined by various developers working on previous Fallout games, even if they also contradicted each other at times.
It should be noted, that thus far Bethesda has not officially ruled on what is canon and what is not. Most of the information on their take is from Fallout 3 in-game material.
- Fallout and Fallout 2 are canon according to Bethesda, as Fallout 3 incorporates the majority of the back story of Fallout and Fallout 2.
- Fallout 3 is Bethesda's production, therefore part of Bethesda's Fallout canon, as are all other games and supplementary materials released by Bethesda, unless stated otherwise.
- Fallout: New Vegas, which was published by Bethesda, is canon, as are all officially released supplementary materials, like the All Roads comic.
- Fallout Tactics is considered semi-canon: major events are canon (and are referenced in Fallout 3), but some details are not. As part of the Fallout Tactics release, Fallout: Warfare has the same level of canonicity.
- While Van Buren (the canceled Fallout 3 by Black Isle Studios) is not officially canon, some elements of it were incorporated into Fallout 3 and its add-ons, as well as into Fallout: New Vegas, and are now part of the Fallout canon.
- The Fallout Bible by Chris Avellone is considered canon as long as it doesn't contradict the canon games, and some setting elements introduced in the Bible have been further developed in Fallout 3 and Fallout: New Vegas. Some erroneous elements of the Bible were later corrected either in following issues of the Bible, in the games or by the developers themselves.
- Fallout: Brotherhood of Steel has been declared as non-canon by Bethesda Softworks, though some elements do not contradict it.
- Canceled games that were not developed by Black Isle Studios, such as Fallout Extreme, Fallout Tactics 2 and Fallout: Brotherhood of Steel 2 are non-canon.
- While there has not been any official statement on this matter, Interplay's Fallout Online is likely considered non-canon by Bethesda due to the two companies' legal dispute. However, due to Interplay claiming to own the franchise again, the game may be canon.