Half-Life is a 1998 first-person shooter (FPS) game developed by Valve and published by Sierra Studios for Windows. It was Valve's debut product and the first game in the Half-Life series. Players assume the role of Gordon Freeman, a scientist who must escape the Black Mesa Research Facility after it is invaded by aliens. The gameplay consists of combat, exploration, and puzzle-solving.

Unlike other games of the time, the player has almost uninterrupted control of the player character, and the story is told mostly in scripted sequences rather than cutscenes. Valve co-founder Gabe Newell said that the team was disappointed with the lack of innovation in the FPS genre, and aimed to create an immersive world rather than a "shooting gallery". Valve developed using GoldSrc, a heavily modified version of the Quake engine, licensed from id Software. The science fiction novelist Marc Laidlaw was hired to shape the story and assist with design.

Half-Life received acclaim for its graphics, gameplay, and narrative, and won over 50 PC "Game of the Year" awards. It is considered one of the most influential FPS games and one of the best video games ever made. By 2008, it had sold over nine million copies. It was followed by the expansion packs Opposing Force (1999) and Blue Shift (2001), developed by Gearbox Software. It was ported to the PlayStation 2 in 2001, along with the expansion Half-Life: Decay, and to OS X and Linux in 2013. Valve ported Half-Life to its Source engine as Half-Life: Source in 2004. In 2020, Crowbar Collective released an unofficial remake, Black Mesa.

Half-Life inspired numerous fan-made mods, some of which became standalone games, such as Counter-Strike, Day of Defeat and Sven Co-op. It was followed by Half-Life 2 (2004), Half-Life 2: Episode One (2006), Half-Life 2: Episode Two (2007) and Half-Life: Alyx (2020).