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, $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 to the Xbox, Xbox 360, and PlayStation 3 video game consoles.

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 the extraterrestrial civilization known as the Combine. The game takes place in and around the fictional City 17 and follows the adventures of scientist Gordon Freeman. Freeman must fight against increasingly unfavorable odds in order to survive.

Half-Life 2 garnered near-unanimous positive reviews and received critical acclaim, winning over 35 PC 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 December 3, 2008, over 6.5 million copies of Half-Life 2 have been sold at retail. Although Steam sales figures are unknown, their rate surpassed retail's in mid-2008 and they are significantly more profitable per-unit.

According to GameSpy, Half-Life 2 is the fifth most played online computer game (excluding MMOGs).

Gameplay

Since Half-Life 2 is a first-person shooter, the gameworld is always presented from Gordon Freeman's perspective, and the player is implicitly asked to place himself or herself in Gordon's shoes. A heads-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 replenished by picking up medical supplies and energy cells respectively, or by using wall-mounted charging devices.

The player defeats enemies with an assortment of weapons. The game's available arsenal consists largely of modern-day weapons, primarily pistols and shotguns, though more elaborate weapons are available, such as a crossbow that shoots hot metal rods, and a pheromone pod that summons alien creatures. The Source engine's physics capabilities extend into combat via a special device called the Zero-Point Energy Field Manipulator, or "gravity gun." Using this device, the player can pick up objects and either hurl them at enemies or hold them in place to create a makeshift shield. The gravity gun can also perform a variety of non-combat functions, such as grabbing out-of-reach supplies, forming bridges across gaps, and flipping overturned vehicles.