Every month, Microsoft adds new feature updates to the Xbox One platform, and an upcoming one is going to be a doozy: backward-compatibility with the Xbox 360.

“Or goal is to deliver the largest games catalog ever on Xbox, and Xbox backwards compatibility allows us to do just that,’’ said Microsoft’s Xbox chief, Phil Spencer at the company’s E3 press conference.

Mike Ybarra, head of the platform engineering team at Microsoft, said that Microsoft will offer “hundreds of titles by the holiday season, with hundreds more in the months to come.” Digital versions of your Xbox 360 titles will automatically begin showing up on Xbox One, he said.

Retails discs will work as well: “Put the disc in, download the game, and you’re ready to go,” Ybarra said.

Microsoft showed off Mass Effect running on the Xbox One, but with Xbox One features, such as broadcasting, game DVR, and the ability to take screenshots or stream via the Xbox app on Windows 10. You’ll also be able to play multiplayer with your Xbox 360 friends.

And apparently all that’s required is for the developer to sign off on it. “No work is required on their part to make all of this possible,” Ybarra said.

If you’re an Xbox Preview member, you’re lucky—you’ll get an initial set of titles try out today, Ybarra said. For everyone else, there’s the holidays.

“For Xbox One, we won’t charge you to play the games you already own,” Ybarra said. That was obviously a dig at Sony, whose PlayStation Now service allows you to play PlayStation 3 games on your PlayStation 4 console, but charges you $20 per month for the service.

Why this matters: Backward-compatibility was one feature that every Xbox One gamer hoped for, but no one actually expected. Now, any Xbox One owner can take advantage of their back catalog—or jump in to older games that they may have passed over. Game developers have to be excited—Microsoft just gave its Xbox 360 lineup a jumpstart, opening those older titles up to gamers who owned a PlayStation during that generation.