Portal 2 is a first-person action/puzzle game, developed by Valve Corporation. It is the sequel to the critically acclaimed 2007 video game Portal and was announced on March 5, 2010, following a week-long alternate reality game based on new patches to the original game. Though initially slated for release in the last quarter of 2010, the game was postponed to the week of April 18, 2011. The game was released by Valve through Steam for Windows and Mac OS X, while the PlayStation 3, Xbox 360 and retail Windows/OS X versions of the game are distributed by Electronic Arts. All versions of the game became available simultaneously.
In Portal 2 the player returns as the human Chell, having been in stasis for several hundred years while GLaDOS (Genetic Lifeform and Disk Operating System), a rampant artificial intelligent computer, and the rest of the Aperture Science facility has fallen into disrepair. Chell is awakened by one of GLaDOS's personality cores, and inadvertently wakes GLaDOS; GLaDOS is immediately displeased at Chell's return and begins testing her again through numerous chambers as she rebuilds the dilapidated facility. Portal 2 continues to challenge the player through numerous platforming and physics-based puzzles using the Aperture Science Handheld Portal Device (the "portal gun"), a device that can create a temporary wormhole between almost any two flat surfaces. Other gameplay elements are added to Portal 2 including tractor beams, laser redirection, and special paint-like gels, taken from the Independent Games Festival-winning DigiPen student project Tag: The Power of Paint, that imparts special properties to objects it touches, such as increased momentum. Portal 2 also includes a two-player co-operative mode in which each player-character is a robot that has become self-aware due to GLaDOS' influence, and requires the players to work together with their own portal guns to complete each level.