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 Software Corporation and was released on November 16, 2004, following a protracted five-year, $40 million development cycle during which the game's source code was leaked to the Internet. The game uses the Source game engine, which includes a heavily modified version of the Havok physics engine. Originally available only for Windows-based personal computers, the game has since been ported onto the Xbox, Xbox 360, and PlayStation 3 video game consoles.

Taking place in and around the fictional City 17, Half-Life 2 follows the adventures of scientist Gordon Freeman. Dr. Freeman is thrust into a dystopian environment in which the aftermath of the events in Half-Life have come to bear fully upon human society. Freeman is forced to fight against increasingly unfavorable odds in order to survive. In his struggle, he is joined by various allies, including former Black Mesa colleagues, oppressed citizens of City 17, and the Vortigaunts.

Half-Life 2 garnered near unanimous positive reviews and received critical acclaim, winning over 35 Game of the Year awards for 2004. The game has been critically praised for its advances in computer animation, sound, narration, computer graphics, artificial intelligence (AI) and physics. As of June 8, 2006, over four million copies of Half-Life 2 have been sold. Exact numbers for digital delivery service Steam and retail have not been revealed, but in general, the former accounts for 25% of Valve's business and is significantly more profitable per unit. As of July 14, 2006, the Half-Life franchise has sold 16 million units. According to GameSpy, Half-Life 2 is the second most played online computer game (excluding MMOGs), surpassed only by Half-Life.

Gameplay

Gameplay in Half-Life 2 is very similar to that of its predecessor. The player navigates through a linear set of levels, combating transhuman troops, known as the Combine, and hostile alien creatures. Puzzles and sequences involving vehicles are interspersed throughout the game, breaking up moments of fighting.

Like other first person shooters, navigation and combat is presented entirely from a first-person perspective. A head-up display at the bottom of the screen shows the player's health, energy, and ammunition status, while a toggle screen shows available weapons at the top. Health and energy can be revitalized by picking up medical supplies and energy cells, or by utilizing wall mounted charging devices.