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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 its 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. Science fiction novelist Marc Laidlaw was hired to shape the story and assist design..

Half-Life received acclaim for its graphics, realistic gameplay and seamless 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 a 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). Half-Life is a first-person shooter that requires the player to perform combat tasks and puzzle solving to advance through the game. Unlike most of its peers at the time, Half-Life uses scripted sequences, such as a Vortigaunt ramming down a door, to advance major plot points. Compared to most first-person shooters of the time, which relied on cut-scene intermissions to detail their plotlines, Half-Life's story is told mostly using scripted sequences (bar one short cutscene), keeping the player in control of the first-person viewpoint. In line with this, the player rarely loses the ability to control the player character, who never speaks and is never actually seen in the game; the player sees "through his eyes" for the entire length of the game. Half-Life has no "levels"; it instead divides the game into chapters, whose titles flash on the screen as the player moves through the game. Progress through the world is continuous, except for short pauses for loading

The game regularly integrates puzzles, such as navigating a maze of conveyor belts or using nearby boxes to build a small staircase to the next area the player must travel to. Some puzzles involve using the environment to kill an enemy, like turning a valve to spray hot steam at their enemies. There are few bosses in the conventional sense, where the player defeats a superior opponent by direct confrontation. Instead, such organisms occasionally define chapters, and the player is generally expected to use the terrain, rather than firepower, to kill the boss. Late in the game, the player receives a "long jump module" for the HEV suit, which allows the player to increase the horizontal distance and speed of jumps by crouching before jumping. The player must rely on this ability to navigate various platformer-style jumping puzzles in Xen toward the end of the game. For the most part, the player battles through the game alone, but is occasionally assisted by non-player characters; specifically security guards and scientists who help the player; the guards will fight alongside the player, and both guards and scientists can assist in reaching new areas and impart relevant plot information. An array of alien enemies populate the game, including headcrabs, bullsquids, and headcrab zombies. The player also faces human opponents including the Hazardous Environment Combat Unit (HECU) Marines and black ops assassins. Half-Life includes online multiplayer support for both individual and team-based deathmatch modes.