Fallout 5 will exist "in a world where the stories and events of the show happened or are happening," says Todd Howard

Fallout 5 will be canon to the events that happen in Amazon's TV adaptation of Bethesda's video games.

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source https://www.eurogamer.net/fallout-5-will-exist-in-a-world-where-the-stories-and-events-of-the-show-happened-or-are-happening-says-todd-howard