The Xbox Series X is backward compatible with nearly every Xbox One game, hundreds of Xbox 360 games, and a handful of original Xbox games. We've known that for some time, but now Microsoft has given us some details on just how ambitious its backward compatibility plans are.

The Xbox Series X will not only have the ability to run older games, it will have the ability to run games at higher resolutions and faster frame rates than could be achieved on older systems. To achieve this, Microsoft invested over 100,000 hours of testing.

This information comes from a blog post on the official Xbox website, written by Jason Ronald, Director of Program Management for the Xbox Series X: Backwards compatibility will be available on launch day and will include thousands of titles, including over 500 Xbox 360 games.

The Xbox Series X should theoretically be compatible with any Xbox One game, but Microsoft has spent considerable time testing games to ensure compatibility.

"With over 100,000 hours of playtesting already completed, thousands of games are already playable on the Xbox Series X today, from big titles to cult classics to fan favorites," Ronald wrote. Many of us at Team Xbox play Xbox Series X as our primary console every day, and the switch between generations is seamless. By the time we launch this holiday season, the team will have put in over 200,000 hours to make sure our game library is ready to jump right in."

But potentially more interesting is Ronald's claim that these older games look better than ever and play more smoothly.

"Backward compatible games will run natively on Xbox Series X hardware, running at full CPU, GPU, and SSD power. There is no boost mode, no downclocking, and all backwards compatible games will run at full power on the Xbox Series X. It also creates a whole new class of innovation, including the ability to double the frame rate of some titles from 30fps to 60fps or 60fps to 120fps."

Does this mean that all Xbox Series X backward compatible titles will run at 4K resolution and 120fps, given that most titles developed for the Xbox Series X target 4K and 60fps? Probably not. However, some older games can get very high resolutions, and some older games can get very fast frame rates, which is pretty impressive considering that many of them were up to 1080p, 30fps on their native systems.

Ronald also said that the Xbox team is eager to hear from fans about which titles they would like to see optimized. He said that not all favorite titles will make it to the Xbox Series X because backward compatibility is "a complex mix of technical and licensing challenges."