In 2004, Valve released Half-Life 2 which was developed using their new game engine, and features a more dystopian setting, stronger focus on characters, and implementing physics-based gameplay.

Set twenty years after the events of Half-Life, players control Freeman in joining a resistance to liberate humanity from an alien force known as the Combine.

In 2020, after years of speculation, Valve released Half-Life: Alyx, which was developed exclusively for virtual reality headsets.

It is a prequel set five years before Half-Life 2, where players control Freeman's eventual ally Alyx Vance in her quest to rescue her father from Combine forces and uncover their mysterious "super-weapon".

The Half-Life series is recognized by critics and industry experts for producing some of the most influential first-person shooter games for the genre, both of which have been highlighted for their advancements towards immersive and varied gameplay, level design, storytelling, visuals and sound.

[1] Players control Gordon Freeman, a theoretical physicist at the Black Mesa Research Facility, where an experiment accidentally causes a dimensional rift and triggers an alien invasion.

[2] Unlike many other games at the time, the player has almost uninterrupted control of Freeman, and the story is told mostly through scripted sequences.

[7] Players control US Marine corporal Adrian Shephard, who fights a new group of aliens and black operations units.

[14][15] Blue Shift puts the player in the position of Barney Calhoun, a security guard working at Black Mesa.

The game takes place within the early parts of Half-Life, with Calhoun attempting to escape the facility with a small group of scientists.

[20] Decay focuses on two of Freeman's colleagues, Gina Cross and Colette Green, as the two work with other scientists to counter the effects of the dimensional rift and ultimately attempt to close it.

Released on November 14, 2001, Decay received a weak but overall positive reception from critics, many reviewers stating that it was fun to play through with a friend, but that the game's more puzzle-oriented gameplay detracted from the overall experience.

[26] On October 27, 2005, Valve released Lost Coast, an additional level demonstrating high-dynamic-range rendering (HDR).

[35] In this prequel to Half-Life 2, players control Alyx as she and her father Eli establish the resistance against the Combine in City 17.

[44][45] The Half-Life writer, Marc Laidlaw, opposed the crossover with Portal, feeling it "made both universes smaller", and said later: "I just had to react as gracefully as I could to the fact that it was going there without me.

[48] Set in various locations around the world with little connection to the events of the main Half-Life story, the game is a multiplayer shooter in which players assume the roles of members of combating teams of the governmental counter-terrorist forces and various terrorist militants opposing them.

[52] Black Mesa is a third-party remake of the original Half-Life, developed and published by Crowbar Collective and made in the Source engine.

In the game Magicka there is a playable character (after the addition of a DLC), which closely resembles the original zombie from the Half-Life universe, equipped with a crowbar.

[58] The Half-Life franchise has received critical praise for its in-depth fictional universe, including numerous characters who would go on to become well known in the gaming sphere.

The original Half-Life introduces Gordon Freeman, a theoretical physicist working at the Black Mesa Research Facility who serves as the main silent playable protagonist for the franchise.

Freeman is hired and put into stasis by the G-Man, an enigmatic and questionable businessman with capabilities and powers beyond any ordinary human being.

Other alien species depicted include antlions, human-sized burrowing insectoids; bullsquids, acid-shooting tentacled predators; and barnacles, ceiling-dwelling amorphous creatures who capture others with their sticky tongue.

The developer of the Half-Life series, Valve, was founded in 1996 in Kirkland, Washington by the former Microsoft employees Mike Harrington and Gabe Newell.

Valve began working on the first Half-Life soon after formation, and settled on a concept for a horror-themed 3D action game, using the Quake engine as licensed by id Software.

[59] The success led to its first expansion pack, Half-Life: Opposing Force, which was developed by Gearbox Software, a new company based in Plano, Texas, and announced on April 15, 1999.

It comes packaged with a heavily modified version of the Havok physics engine that allows for an extra dimension of interactivity in both single-player and online environments.

[69] The designer Robin Walker said Valve used Half-Life games to "solve some interesting collision of technology and art that had reared itself".

[71] Walker said Valve failed to find a unifying idea that provided a sense of "wonderment, or opening, or expansion".

[77] A short film, Half-Life: Uplink, was developed by Cruise Control, a British marketing agency, and released on March 15, 1999.

Additionally, The Orange Box, which included Half-Life 2 and both of its episodic expansions, sold 3 million units at retail by November 2008.