Backward compatibility on Xbox One will be a standard feature when it launches later this year. The digital titles that you own and are part of the Back Compat game catalog will automatically show up in the "Ready to Install" section on your Xbox One. For disc-based games that are a part of the Back Compat game catalog, simply insert the disc and the console will begin downloading the game to your hard drive. After the game has finished downloading, you will still need to keep the game disc in the drive to play.

This is the current list of compatible Xbox 360 Games for Xbox One. I tested out Hexic HD last night and it's nice. Full Xbox 360 Guide, Achievements and everything you would expect. More Xbox 360 Games will be added to this list over time.. If you'd like to contribute it's as easy as hitting the reply button!

June 16, 2015:

Xbox One Compatible Xbox 360 Games:
- A Kingdom for Keflings
- A World of Keflings
- Alien Hominid HD
- Banjo-Kazooie
- Banjo-Tooie
- BattleBlock Theater
- Defense Grid
- Geometry Wars Evolved
- Hexic HD
- Jetpac Refuelled
- Kameo
- Mass Effect
- N+
- Perfect Dark
- Perfect Dark Zero
- Small Arms
- Super Meat Boy
- Toy Soldiers
- Toy Soldiers: Cold War
- Viva Piñata
- Viva Piñata: TIP
- Zuma

Coming Soon (confirmed):
- Rainbow Six Vegas
- Rainbow Six Vegas 2
- Fallout 3

Source: http://www.xboxone-hq.com/news/529-e3-2015-xbox-one-backwards-compatibility-out-now-for-preview-members.html

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I played Kameo the other day and had to download a 4-5GB update.. In otherwords, you are basically downloading the whole game. But it works well, i played for quite a while.

I dunno if it's just me getting a new TV and all, but I did notice a touch more clarity playing these old Xbox 360 games through the Xbox One. Am I the only one?