Half-Life (stylized as HλLF-LIFE) is a science fiction video game developed by Valve Corporation, the company's debut product and the first in the Half-Life series. First released in 1998 by Sierra Studios for Windows PCs, the game was also released for the PlayStation 2; Mac OS X and Linux ports became available in January 2013. In Half-Life, players assume the role of Dr. Gordon Freeman, a theoretical physicist who must fight his way out of a secret underground research facility whose research and experiments into teleportation technology have gone disastrously wrong.

Valve, set up by former Microsoft employees, had difficulty finding a publisher for the game, with many believing that it was too ambitious a project. Sierra On-Line eventually signed the game after expressing interest in making a 3D action game. The game had its first major public appearance at the 1997 Electronic Entertainment Expo. Designed for Windows, the game's engine, GoldSrc, was a heavily modified version of id Software's Quake game engine with code portions from the id Tech 2 engine.

On its release, the game received universal acclaim, with critics praising the seamlessly flowing narrative, presentation and realistic gameplay, and it won over fifty PC Game of the Year awards. Its gameplay influenced the design of first-person shooters for years after its release, and it is widely considered to be one of the greatest games of all time. IGN ranked Half-Life as the number one greatest first-person shooter of all time, stating that "When you look at the history of first-person shooters, it all breaks down pretty cleanly into pre-Half-Life and post-Half-Life eras." Half-Life had sold eight million copies by 16 November 2004, and 9.3 million copies by December 2008. By 14 July 2007, the Half-Life franchise as a whole had sold over 20 million units. Half-Life was followed by the 2004 sequel Half-Life 2, which also received universal critical acclaim.