Half-Life 2 is a science fiction first-person shooter computer game and the sequel to the highly acclaimed Half-Life. It was developed by Valve Corporation and was released on November 16, 2004, following a protracted five-year,[6] $40 million[7] development cycle during which the game’s source code was leaked to the Internet.[8] The game uses the Source game engine, which includes a heavily modified version of the Havok physics engine and was also the first video game to require online product activation.[9][10] Originally available only for Windows-based personal computers, the game has since been ported to the Xbox and Mac OS X. The game was a part of The Orange Box compilation for Xbox 360 and PlayStation 3 video game consoles.[11]

Half-Life 2 takes place in a dystopian world in which the events of Half-Life have fully come to bear on human society, which has been enslaved by a multidimensional empire known as the Combine. The game takes place in and around the fictional City 17 and follows the adventures of scientist Gordon Freeman who must fight against increasingly unfavorable odds in order to survive.

The game garnered near-unanimous positive reviews and received critical acclaim,[12][13] receiving praise for its advances in computer animation, sound, narration, computer graphics, artificial intelligence (AI), and physics simulation. The game won over 40 PC Game of the Year awards for 2004,[14] and several publications have since named it as game of the decade.[15][16][17][18] As of December 3, 2008, over 6.5 million copies of Half-Life 2 have been sold at retail.[19] Although Steam sales figures are unknown, their rate surpassed retail in mid-2008[20] and are significantly more profitable per-unit.[19]

Two episodic additions to the game, Half-Life 2: Episode One, and Half-Life 2: Episode Two, were released (respectively) in June 2006 and October 2007 as a continuation of the storyline and the events following the original Half-Life 2.