The player assumes the role of Gordon Freeman, a scientist who must escape from the Black Mesa Research Facility after it is overrun by alien creatures following a disastrous scientific experiment.

Half-Life inspired numerous fan-made mods, some of which became standalone games, such as Counter-Strike, Day of Defeat, and Sven Co-op.

[4] 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.

[6] At the underground Black Mesa Research Facility, physicist Gordon Freeman participates in an experiment on a crystal of unknown origin.

Venturing to the surface, Freeman discovers government soldiers have been dispatched to cover up the incident by killing humans and aliens alike.

Escaping through a waste treatment complex, he discovers a laboratory filled with alien specimens, collected long before the resonance cascade.

[9] They did not want to build their own game engine, as this would have created too much work for a small team and Newell planned to innovate in different areas.

[11] Half-Life was inspired by the FPS games Doom (1993) and Quake (1996),[12]: 9, 11  Stephen King's 1980 novella The Mist, and a 1963 episode of The Outer Limits titled "The Borderland".

Sierra On-Line signed Valve for a one-game deal as it was interested in making a 3D action game, especially one based on the Quake engine.

[16] Sierra gave Valve an advance of around $1 million in exchange for 30% of the revenue and 100% of the intellectual property; the rest of development was funded by Newell and Harrington.

[20] When the rest of the team played the level, which the designer Ken Birdwell described as "Die Hard meets Evil Dead", they agreed to use it as a baseline.

Second, the level responded to any player action, even for something as simple as adding graphic decals to wall textures to show a bullet impact.

Instead, Valve created the "cabal", initially a group of six individuals from across all departments that worked primarily for six months straight in six-hour meetings four days a week.

[29] Half-Life was censored in Germany to comply with the Federal Department for Media Harmful to Young Persons, which regulates depictions of violence against humans.

Valve replaced the human characters with robots, spilling oil and gears instead of blood and body parts when killed, among other changes.

[31] Rebecca Heineman, the co-founder of Logicware, denied this, saying that Valve cancelled the port as Apple had angered them by misrepresenting sales projections.

[33] Captivation Digital Laboratories and Gearbox Software developed a port of Half-Life for the Dreamcast, with new character models and textures and an exclusive expansion, Blue Shift.

[40] Black Mesa, a third-party remake of Half-Life developed by Crowbar Collective on the Source engine, was published as a free mod in September 2012 and later approved by Valve for a commercial standalone release.

[41][42] In November 2023, for the 25th anniversary of Half-Life, Valve updated the Steam version to revert content to its original 1998 state, fix long-standing bugs, and add content including the Half-Life: Uplink demo, four new multiplayer maps, Steam Deck support, rendering improvements, and support for 4K resolution monitors.

[43][44][45] Valve also released an hour-long documentary on the creation of Half-Life, featuring commentary from the original developers, designers and artists.

[48] Gearbox developed a second expansion pack, Blue Shift, in which players control Barney Calhoun, a security guard at Black Mesa, as he attempts to escape the facility.

[50][51] Gearbox created a cooperative multiplayer expansion pack, Decay, exclusively for the PlayStation 2 port of Half-Life which is played through the perspectives of Gina Cross and Colette Green, two Black Mesa scientists.

[53] Other mods such as Counter-Strike and Day of Defeat (DOD) began life as the work of independent developers who later received aid from Valve.

Single-player mods include USS Darkstar (1999, a futuristic action-adventure on board a zoological research spaceship) and They Hunger (2000–2001, a survival horror total conversion trilogy involving zombies).

Team Fortress Classic, Day of Defeat, Gunman Chronicles (2000, a futuristic Western movie-style total conversion with emphasis on its single-player mode) and Sven Co-op were also released as standalone products.

[69][70] Wired's Julie Muncy called the Xen sequence "an abbreviated, unpleasant stop on an alien world with bad platforming and a boss fight against what appeared, by all accounts, to be a giant floating infant".

[66] The PlayStation 2 version was a nominee for The Electric Playground's 2001 Blister Awards for "Best Console Shooter Game", but lost to Halo: Combat Evolved for Xbox.

[82] In 2007, IGN described Half-Life as one of the most influential video games,[83] and in 2013 wrote that the history of the FPS genre "breaks down pretty cleanly into pre-Half-Life and post-Half-Life eras".

[124] Edge noted in 2003 that "a significant number of the 7.5m copies of the PC version were bought because the game offered such potential for community-driven expansion".

The player controls Freeman 20 years after the Black Mesa incident in the dystopian City 17, where he joins a rebellion against an alien regime.