Today during Microsoft's E3 press conference, Xbox head Phil Spencer confirmed Xbox One will support backward compatibility with over 100 select Xbox 360 games by this holiday season.
Digitally purchased games will start showing up automatically in your game library. Retail Xbox 360 disks won't work natively with the Xbox One, but will initiate a download of an Xbox One-compatible version when inserted. During the announcement, the Xbox 360 version of the original Mass Effect was shown running on the Xbox One.
Developers and publishers just need to approve what titles work with backwards compatibility, and Microsoft will do the rest, said Spencer.
Xbox One backward compatibility is available now for preview members and everyone this holiday. 
Brandin Tyrrel is an Associate Editor at IGN. You can follow him on Twitter at @BrandinTyrrel.