Microsoft early on said Xbox Series S and X would be backwards compatible with the games playable on Xbox One, which means quite a few Xbox 360 titles and even some from the original Xbox. Now they have done +500K hours of testing and have revealed exactly which games that are compatible with Xbox Series X - and it is all of them except Kinect titles.
They also remind us that these games will "look and play better" on the new console, so it's a great opportunity to return to old favourites if to scratch that retro itch.