Microsoft announced Xbox One backward compatibility at E3 in 2015.
The announcement came during Microsoft's E3 2015 press conference. It revealed that selected Xbox 360 games would run on Xbox One through a software-based compatibility system, allowing players to use supported older titles on the newer console.
The feature initially entered preview testing through the Xbox One Preview Program. Microsoft later made the first public backward-compatible games available in November 2015. Compatibility was added title by title because games required testing and technical adjustments rather than simply running from any Xbox 360 disc.
This announcement is sometimes confused with Xbox One's later support for original Xbox games, which Microsoft introduced in 2017. It is also distinct from Smart Delivery, a later system for obtaining the appropriate version of selected Xbox One and Xbox Series games.