Half-Life

Half-Life, often abbreviated HL, is a science fiction first-person shooter computer game developed by Valve Software and published by Sierra Studios. Released on November 19, 1998 for PCs running Microsoft Windows, the game was based on a heavily modified version of the Quake game engine. With eight million copies sold since release, Half-Life is the best selling PC first-person shooter to date. The game was later released for PlayStation 2 on November 15, 2001, and a Sega Dreamcast port was planned, but cancelled as the machine's popularity waned.

In Half-Life, players assume the role of Dr. Gordon Freeman, a theoretical physicist, who must fight his way out of a research facility overrun by aliens and soldiers. The game was the first first-person shooter to feature a story told entirely in-game and in real time, without the use of cutscenes. Heralded by critics for its presentation and numerous scripted sequences, Half-Life won over 50 Game of the Year awards, and was named "Best PC Game Ever" by PC Gamer in its November 1999, October 2001, and April 2005 issues. The game influenced the development of first-person shooters in the years following its release, and is often cited as being revolutionary in its genre.

Half-Life's success continued for years with expansions such as Half-Life: Opposing Force and the standalone Half-Life: Blue Shift, mods such as Counter-Strike, Team Fortress Classic, Deathmatch Classic, Ricochet and Day of Defeat, and its sequel Half-Life 2. The Half-Life franchise, including Counter-Strike and Day of Defeat, has seen over 15 million sales.

Single Player
The game is set in a remote area of New Mexico at the Black Mesa Research Facility, a fictional complex that bears many similarities to both the Los Alamos National Laboratory and Area 51, during May of 200X, meaning it takes place sometime between the years 2000 and 2009. The game's protagonist is the theoretical physicist Gordon Freeman, a survivor of an experiment that goes horribly awry when an unexpected resonance cascade (an apparently completely fictitious occurrence; see also 1, 2) rips dimensional seams that devastate the facility. Aliens from another world - known as Xen - subsequently enter the facility through these dimensional seams.

As Freeman tries to make his way out of the ruined facility to find help for the injured, he soon discovers he is caught between two sides: the hostile aliens, and the Hazardous Environment Combat Unit, a military force dispatched to cover up the incident — including eliminating Freeman and the rest of the scientists. Throughout the game, a mysterious figure known as (but not actually referred to in-game as) the G-Man regularly appears, apparently monitoring Freeman's progress. Ultimately, Freeman uses the co-operation of surviving scientists and security officers to work his way to the mysterious "Lambda Complex" of Black Mesa, where a team of survivors teleport him to the alien world Xen, where he must kill the Nihilanth, the creature keeping Xen's side of the dimensional rift open.

The game's plot was originally inspired by the computer games Doom and Quake, both produced by id Software, Stephen King's short story/novella The Mist, and an episode of The Outer Limits called "The Borderland." It was later developed by Valve's in-house writer and author, Marc Laidlaw, who wrote the books Dad's Nuke and The 37th Mandala. However, the more influential aspect of the single player mode is not the plot itself, but rather how it is presented to the player.

The game tells the story by flowing into scripted sequences that are integrated as part of the game rather than as cut scene intermissions. These sequences range from the introduction of major plot points such as the resonance cascade to bringing the player into a particularly difficult part of a level. Two of the intended results of this style of presentation were to increase immersion and to maintain a contiguous narrative that keeps the player's interest.

Valve implemented other factors to heighten the feeling of immersion, including that the player never sees or hears their own character, who remains a 'silent protagonist' in the plot, and that the player rarely loses the ability to control Gordon, even during monologues. The scripted sequences help flow by keeping the player in the game, whereas cutscenes in other contemporary games had often been a diversion from previous segments of their gameplay. The levels for HL were also divided into small sections to minimize long interruptions from loading.

Storyline
Dr. Gordon Freeman starts his day riding a tram deep into the heart of the Black Mesa Facility on his way to the Anomalous Materials Lab to begin work. Nothing seems to be out of place, except a chemical leak which forces the tram to momentarily stop. It is at that point that Gordon notices a strange man in a blue suit watching him from another tram. Later on, once he reaches his destination, Gordon once again sees the suited man arguing heatedly with a scientist behind soundproof glass. After donning his HEV suit he proceeds to the test chamber. After pushing an alien sample into the scanning beam, immediately something goes terribly wrong. A "resonance cascade" has been triggered, creating an interdimensional portal between Earth and a bizarre world called Xen. Gordon is briefly teleported there during the ordeal, at one point coming into contact with what looks like sentient life forms.

Finding himself back in Black Mesa, Gordon is told that all communication outside Black Mesa has been lost, and decides to try and make it to the surface to get help. Black Mesa has received massive structural damage, and to make matters worse, aliens from Xen begin randomly teleporting all around them. Most are simply reacting naturally to their new surroundings, but even more hostile are the creatures Gordon saw earlier (known as Vortigaunts), who seem to attack with a single minded intent to kill. Also strange is that Gordon occasionally finds the man in the suit, the G-Man, watching him from out-of-reach places, always walking off before Gordon can get to him. Gordon makes good progress until he learns that the military is already on the way. He attempts to join up with them, but walks into a room to witness the Marines kill a group of survivors. It is then that he discovers that the Hazardous Environment Combat Unit, which has taken control of Black Mesa, isn't just killing the aliens, they're also killing everyone connected to the project. However, it is revealed that the surviving members of Lambda Team (in the Lambda Complex at the other end of the facility) may be able to close the dimensional rift; Gordon begins the journey across Black Mesa to reach them.

From here, Gordon has several adventures, such as killing a giant, rapidly growing creature, riding across the facility on a railway system in order to reach a satellite rocket that must be launched in order to reverse the resonance cascade, and fighting a group of mysterious Black Ops, before being captured by marines and dumped in a garbage compactor. However, Gordon is able to escape and eventually winds up in a secret part of the Facility where he discovers that scientists had been "collecting" specimens from Xen long before this whole mess occurred.

After arming himself with advanced prototype weaponry, Gordon reaches the surface. It has become a warzone. The Vortigaunts, along with a new, soldier type enemy, and a giant monstrosity, have begun fighting the marines who are beginning to lose. They've called in reinforcements, but it isn't enough. Gordon must scale cliffs and navigate the bombed out buildings while avoiding both sides as the forces of Xen begin to dominate the battlefield. Finally, Gordon reaches relative safety underground.

The Marines begin to evacuate Black Mesa and airstrikes begin. Gordon navigates underground water channels and tries to avoid scores of alien soldiers as they pick off remaining marine stragglers. After much struggle, Gordon finally reaches the Lambda Complex, where he discovers that the Lambda Complex is where they developed the teleportation technology that allowed scientists to travel to Xen in the first place. The handful of surviving personnel, who have holed themselves up in a small stronghold, inform Gordon that, unfortunately, the satellite he launched was not able to reverse the effects of the resonance cascade because an immensely powerful being on the other side is keeping the portal between the worlds open, and Gordon must kill it to prevent the Xen aliens from taking over completely. They activate the teleporter and Gordon is relocated to Xen.

On the strange borderworld, Gordon encounters many of the aliens that had been brought into Black Mesa, as well as the remains of HEV-wearing researchers that came before him. After killing the mighty Gonarch, and fighting his way through an alien camp, Gordon arrives at what seems to be a genetic flesh factory. The forces of Xen are literally assembling their own army to conquer Earth. After fighting his way through mysterious, levitating creatures, he finds a giant portal and enters it.

In a vast cave, Gordon finally confronts the powerful and mysterious being that is holding the portal open: the Nihilanth. Gordon destroys the being's shield, allowing him to destroy its vulnerable brain. As the creature dies, it floats toward the ceiling, and explodes in a giant green blast that overpowers Gordon's senses. When he comes to, he is confronted by the G-Man. Both are transported to various locales around Xen, while the G-Man praises Gordon's actions in the border world (which he refers to as "a nasty piece of work"). He explains that his "employers", agreeing that Gordon has "limitless potential", have authorised him to offer Freeman a job. The final teleportation takes Gordon to the same tram car that he rode on at the beginning of the game, but apparently flying through space at immense speed. Gordon accepts this offer by stepping though a portal, and, finding himself floating in nothingness, hears the G-Man's voice one last time: "Wisely done, Mr. Freeman, I will see you up ahead..." The story continues from this point in Half-Life 2.

Alternatively, the player may choose not to step into the portal in the final scene. This causes Gordon to be teleported into a giant cave facing rows of Alien Grunts as the game ends with the G-Man's comment, "Well, it looks like we won't be working together. No regrets, Mr. Freeman." Gordon's appearance in Half-Life 2 suggests that this "bad ending" is not part of Half-Life canon.

Weapons
There are 14 weapons available to players in both single player and multiplayer games of Half-Life. Half-Life: Opposing Force added several more weapons. Many reviews of Half-Life mentioned the impressive functionality and "usefulness" of all the weapons designed. Each weapon's damage profile is distinct, none feeling superfluous or excessively powerful; each has a specific advantage in the appropriate situation. The weapons in HL (without expansions) are:


 * Crowbar: A simple melee weapon that is iconic of Gordon Freeman and Half-Life.
 * Pistol (GLOCK 17, or Beretta 92F/FS with High Definition Pack).
 * .357 Magnum (Colt Python revolver).
 * Submachine gun (Heckler & Koch MP5N, or M16 converted to 9mm with High Definition Pack, with mounted M203 grenade launcher).
 * Shotgun (Franchi SPAS-12).
 * Crossbow: The only sniper weapon in Half-Life.
 * Hornet gun (alien weapon): Also known as the "Hivehand". The same weapon used by the Alien Grunts, this gun is a living creature which appears to be a larva of some sort (Due to the fact that it sports no legs). After firing out the hornets, the creature turns erect & tense, after a short while though, it returns to its normal breathing state.
 * Rocket propelled grenade (RPG) launcher (ATGM-4000 RPG Launcher).
 * Quantum destabilizer (Gluon/Egon gun): This experimental weapon, named "Gluon Gun" by its creator, fires out a concentrated energy which disrupts its target's molecules. It looks and operates similar to the proton pack used by the characters in the movie Ghostbusters. Because of its internal weapon name, weapon_egon, it is also known as the Egon gun; this is probably a reference to the Ghostbusters character Egon Spengler.
 * Tau cannon/Gauss gun: Another experimental weapon that rapidly shoots beams of tau particles that reflect off walls if hit indirectly. Secondary fire allows the gun to charge up to shoot a more powerful beam that can penetrate thin walls and pushes the user in the opposite direction. The recoil is deliberately exaggerated in multiplayer so the player can "Gauss jump" very high and reach hidden areas or escape opponents. This feature is a deliberate nod to "rocketjumping" in Quake. If the gun is kept charged for too long (ca 10s), it overloads and discharges, damaging its wielder.
 * Hand grenade.
 * Laser trip mine: A high-explosive Claymore mine-like device that can be attached to walls. It is set off either by damaging the mine or by "breaking" the laser "tripwire" emitting from it.
 * Satchel charge: A high-explosive that can be thrown a short distance and detonated from a distance.
 * Snarks (alien weapon): Aggressive and small alien creatures that quickly pursue their target, pestering and biting, until finally exploding after several seconds (or if shot). If they cannot locate a hostile target, they will turn on the player that set them loose.

Additionally, the long jump module can increase the horizontal distance and speed of jumps. This increased mobility can be used to dodge attacks quickly and jump from one platform to another. The long jump is done by moving forward, crouching, then jumping.

Development
Half-Life was the first product for Kirkland, Washington-based developer Valve Software, which was founded in 1996 by former Microsoft employees Mike Harrington and Gabe Newell. They settled on a concept for a horror-themed 3D action game, and licensed the Quake engine from id Software. Valve eventually modified the engine a great deal, notably adding skeletal animation and Direct3D support; a developer later stated that seventy percent of the engine code was rewritten. The company had difficulties finding a publisher at first, many believing their project "too ambitious" for a studio headed by newcomers to the video game industry. Sierra On-Line had been very interested in making a 3D action game, especially one based on the Quake engine, and so signed them for a one-game deal.

The original code name for Half-Life was Quiver, after the Arrowhead military base from Stephen King's novella The Mist, which served as early inspiration for the game. Gabe Newell explained in an interview that the name Half-Life was chosen because it was evocative of the theme, not clichéd, and had a corresponding visual symbol: the Greek letter λ (lower-case lambda), which represents the decay constant in the half-life equation.

The first public appearances of Half-Life came in early 1997; it was a hit at Electronic Entertainment Expo that year, where they primarily demonstrated the animation system and artificial intelligence. Valve Software hired science fiction author Marc Laidlaw in August 1997 to work on the game's characters and level design. Half-Life was originally planned to be shipped in late 1997, to compete with Quake II, but was postponed when Valve decided the game needed significant revision.

In a 2003 Making Of... feature in Edge, Newell discusses the team's early difficulties with level design. In desperation, a single level was assembled including every weapon, enemy, scripted event and level design quirk that the designers had come up with so far. This single level inspired the studio to press on with the game. As a result, the studio completely reworked the game's artificial intelligence and levels in the year leading up to its release. At E3 1998 it was given Game Critics Awards for "Best PC Game" and "Best Action Game" at the expo. The release date was delayed several times in 1998 before the game was finally released in November of that year.

Ports
Half-Life was ported to the PlayStation 2 by Gearbox Software and released in 2001. This version of the game had a significant overhaul in terms of both character models, weapons, and more advanced and extended levels and general map geometry. Also added in was a head-to-head play and a co-op expansion called Half-Life: Decay that allowed players to play as the two female scientists Dr. Cross and Dr. Green at Black Mesa.

Versions for the Sega Dreamcast and Apple Macintosh were essentially completed, but never commercially released.

Dreamcast version
Gearbox Software was slated to release a port to the Sega Dreamcast under contract by Valve and their then publisher Sierra On-Line near the end of 2000. At the ECTS 2000, a build of the game was playable on the publisher's stand, and developers Randy Pitchford and Brian Martel were in attendance to show it off and give interviews to the press. However, despite only being weeks from going gold, it was never commercially released; Sierra announced that Half-Life on Dreamcast was cancelled "due to changing market conditions" (presumably the third-party abandonment of the failing Dreamcast). The following year Sierra On-Line showed a PlayStation 2 port at E3 2001. This version was released in North America in late October of the same year, followed by a European release just a month later. Around the same time, Half-Life: Blue Shift, which was intended to be a Dreamcast exclusive Midquel, was released on PC as the second Half-life Expansion Pack.

Although it has never officially been released, the Dreamcast version was leaked onto the Internet, and was proven to be fully playable; it contains the full versions of Half-Life and Blue Shift, both with an early version of the High Definition Pack (it was from this port that the pack was spawned), but has a somewhat inconsistent framerate (though never to the point of unplayability) and lengthier load times when the player moves from area to area (around ten seconds, while today's average PC can load an area in around one and a half). Also, there are some saving problems; the number of blocks required to save on a VMU increases rapidly as the player reaches the end of a level, then drops at the start of the next. While the game allows you to remove files to increase space, sometimes it still isn't enough.

The console's mouse and keyboard peripherals are supported, if preferred to the standard controller. If the controller is used, the game adds an auto-aim feature, so that when an enemy nears the center of the player's vision, the aiming crosshair will shift over toward the enemy to make shooting them easier. The game's controls are customizable. The game has no multiplayer mode, and lacks the parental feature of the PC version (players cannot turn the gibs off). It does have an interesting password feature, however; with three dials, the user makes various phrases, such as "Otis Loves Dreamcast" (god mode), "Fear and Gravity" (jump to Xen in HL), or "Barney Goes To Work" (skip the intro in Blue Shift and jump right into the main game, pre-resonance cascade).

Macintosh version
Though more or less complete and ready for mass production, the Macintosh port of Half-Life was scrapped because of incompatibility with the Windows PC version's multiplayer mode. The developers also stated that mods for PC Half-Life would not be compatible with the Mac port. Additionally, concerns over the task load associated with providing technical support on more than one end platform at once may have contributed to the demise of Half-Life for Macintosh.

Later developments
The sequel, Half-Life 2, was merely a rumor until it was finally revealed at E3 in May 2003, which ignited a firestorm of hype surrounding the game. The player again takes the role of Gordon Freeman, this time several years after the Black Mesa incident in the dystopic Eastern European "City 17" where he must fight as part of a rebellion against an alien regime. After a series of controversies and delays Half-Life 2 was released on November 16, 2004.

To experience first-hand the processes mod-makers would have to go through with the new engine, Valve ported Half-Life (dubbed Half-Life: Source) and Counter-Strike to their new Source engine. Half-Life: Source is a straight port, lacking any new content or the Blue Shift High Definition pack. However, it does take advantage of vertex and pixel shaders for more realistic water effects, as well as Half-Life 2's realistic physics engine. They also added several other features from Half-Life 2, including an improved dynamic lightmap, vertex map, and shadowmap system with cleaner, higher resolution and specular texture and normal maps, as well as utilization of the render-to-texture soft shadows found in Half-Life 2's Source engine, along with 3D skybox replacements in place of the old 16-bit color bitmap skies. Also redesigned was the crossbow that will pin its targets to a nearby wall (if they're close enough). The Source engine itself, however, is not entirely perfect. Certain control issues are generally regarded as being a problem, such as ladder bounce (where disconnecting from a ladder gives a sideways vector boost, which can be highly undesirable when trying to move near a ladder on a platform over a fall), fall push (where moving off a platform into a fall gives a sideways vector boost), and jumping from platforms (where the graphics engine tends to lead the player to think a jump can be made later than it really can). Naturally, the Half-Life port possesses the Source engine's control weaknesses as well as its many graphical strengths. Half-Life: Source is available with special editions of Half-Life 2. This port has been criticised however, for not utilizing many of the features of the Source engine found in Half-Life 2, as it still used textures and models from the original game. Due to this, a third-party remake called Black Mesa is also under development. Day of Defeat: Source was released on September 26, 2005.

On June 10, 2005, Valve announced through their Steam update news service an upcoming port of Half-Life Deathmatch, the multiplayer portion of the original game, much in the same fashion as the earlier released Half-Life: Source. No exact release date was given, simply the words "In the coming weeks..." On July 2, 2006, Half-Life Deathmatch: Source was released.

Recently, Half-Life 2: Episode One was released. It is part of a three episode trilogy, of which the second episode is scheduled to be released in winter 2006.

Expansions
Two expansion packs by outside developer Gearbox Software have been released for the PC version: Half-Life: Opposing Force (1999) and Half-Life: Blue Shift (2001). The former, often shortened to OpFor or OP4, returns the player to Black Mesa during the events of Half-Life's storyline, but this time from the perspective of the U.S. Marines sent to cover up evidence of the incident. It introduced several new weapons (notably the M249 SAW LMG and a Barnacle grappling gun), new non-player characters, both friendly and hostile (Otis the security guard and the "Race X" aliens, respectively) and new, previously unseen areas of the facility. The expansion is shorter than Half-Life, having 11 chapters to the original's 19.

Blue Shift returns the player to HL's Black Mesa timeline once more, this time as one of the facility's security guards. (This expansion was originally developed as a bonus mission for the canceled Dreamcast version.) Blue Shift came with an optional High Definition Pack that could update the look of Half-Life, Opposing Force, and the new Blue Shift content. In particular, the models' polygon count and texture resolutions were increased, and some changes were made to the in-game sounds, most notably the shotgun. Blue Shift had relatively little new content compared to Opposing Force: aside from a few models (jacket-less scientists and security guards, Otis, and Dr. Rosenberg) all content was already present in the original Half-Life.

Half-Life: Decay was another expansion by Gearbox, released only as an extra with the PlayStation 2 version of Half-Life. The add-on featured cooperative gameplay in which two players could solve puzzles or fight against the many foes in the Half-Life universe.

In 2000, a pack titled the Half-Life Platinum Collection was released, with these games included:


 * Half-Life
 * Counter-Strike
 * Team Fortress Classic
 * Half-Life: Opposing Force

In 2002, the pack was rereleased with Half-Life: Blue Shift included.

Mods
From its release in 1998, Half-Life saw fervent support from independent game developers, due in no small part to support and encouragement from Valve Software. Worldcraft, the level-design tool used during the game's development, was included with the game software. Printed materials accompanying the game indicated Worldcraft's eventual release as a retail product, but these plans never materialised. Valve also released a software development kit, enabling developers to modify the game and create mods. Both tools were significantly updated with the release of the version 1.1.0.0 patch. Many supporting tools (including texture editors, model editors, and rival level editors like the multiple engine editor QuArK) were either created or updated to work with Half-Life.

Half-Life's code has been released and is being used as a base for many multiplayer mods such as the immensely popular and free Counter-Strike. Other popular multiplayer mods include Team Fortress Classic (TFC), Day of Defeat, Deathmatch Classic (DMC), Action Half-Life, Firearms, and Natural Selection. TFC and DMC were developed in-house at Valve Software. Counter-Strike, Day of Defeat, and others that began life as the work of independent developers (self-termed "modders"), later on received aid from Valve. There was even a free team-based multiplayer mod called Underworld Bloodline created to promote the Sony Pictures movie Underworld.

Numerous single player mods have also been created, like USS Darkstar (1999, a futuristic action-adventure onboard of a zoological research spaceship), The Xeno Project 1 and 2 (1999-2005, a two-part mod starting in Xen and again including spaceships), Edge of Darkness (2000, which features some unused Half-Life models), Half-Life: Absolute Redemption (2000, which brings back Gordon Freeman for four additional episodes and another encounter with the G-Man), They Hunger (2000-2001, a survival horror total conversion trilogy involving zombies), Poke 646 (2002, a follow-up to the original Half-Life story with improved graphics) and Xen-Warrior (2002-2004, based on Half-Life: Chronicles, the player controls an Alien Grunt, similar to Point of View in which players take on the role of a Vortigaunt).

Some Half-Life modifications eventually landed on retail shelves. Counter-Strike was the most successful, unexpectedly becoming the biggest selling online game to date and having been released in five different editions: as a standalone product (2000), as part of the Platinum Collection (2000), as an Xbox version (2003) as the single player spin-off, Counter-Strike: Condition Zero (2004), and the newest addition, Counter-Strike: Source, which runs on Half-Life 2's Source engine. Team Fortress Classic, Day of Defeat and Gunman Chronicles (2000, a futuristic Western movie-style total conversion with emphasis on its single player mode) were also released as stand-alone products.

Soundtrack
Half-Life's soundtrack was composed by Kelly Bailey.


 * 1) "Adrenaline Horror" - 02:09
 * 2) "Vague Voices" (Black Mesa Inbound) - 02:11
 * 3) "Klaxon Beat" - 01:00
 * 4) "Space Ocean" (Echoes of a Resonance Cascade) - 01:36
 * 5) "Cavern Ambiance" (Zero Point Energy Field) - 01:39
 * 6) "Apprehensive Short" - 00:23
 * 7) "Bass String Short" - 00:08
 * 8) "Hurricane Strings" (Neutrino Trap) - 01:33
 * 9) "Diabolical Adrenaline Guitar" (Lambda Core) - 01:44
 * 10) "Valve Theme [Long Version]" (Hazardous Environments) - 01:22
 * 11) "Nepal Monastery" - 02:08
 * 12) "Alien Shock" (Biozeminade Fragment) - 00:36
 * 13) "Sirens in the Distance" (Triple Entanglement) - 01:12
 * 14) "Nuclear Mission Jam" (Something Secret Steers Us) - 02:00
 * 15) "Scared Confusion Short" - 00:16
 * 16) "Drums and Riffs" (Tau-9) - 02:03
 * 17) "Hard Technology Rock" - 01:40
 * 18) "Steam in the Pipes" (Negative Pressure) - 01:55
 * 19) "Electric Guitar Ambiance" (Escape Array) - 01:24
 * 20) "Dimensionless Deepness" (Dirac Shore) - 01:24
 * 21) "Military Precision" - 01:20
 * 22) "Jungle Drums" - 01:49
 * 23) "Traveling Through Limbo" (Singularity) - 01:17
 * 24) "Credits / Closing Theme" (Tracking Device) - 01:39
 * 25) "Threatening Short" (Xen Relay) - 00:37
 * 26) "Dark Piano Short" - 00:17
 * 27) "Sharp Fear Short" - 00:08

(Note: Most of the tracks were retitled and carried over to the Half-Life 2 original soundtrack; the names in parentheses are the revised titles used in the Half-Life 2 OST. Tracks 2, 12, 13, and 24 were remixed for the sequel.)

Trivia

 * Director Quentin Tarantino (who is said to be a Half-Life fan) has reportedly considered the possibility of directing a movie adaptation. Valve has stated however that they have had no contact with Quentin Tarantino nor any of "his people." Valve went on to comment that, "We've had many conversations with folks in Hollywood, but have no commitments for a HL movie at this time." Also, many hollywood scripts for a Half-Life movie have been sent to Valve, but turned away, as Valve said they all "sucked".
 * The Half-Life series draws some inspiration from various adaptations of H. G. Wells's The War of the Worlds; the Vortigaunts who appear throughout the series are almost identical to the Mor-Tax aliens from the War of the Worlds TV series.
 * Half-Life, as well as its two expansion packs Opposing Force and Blue Shift, are all double entendres, named after scientific terms. "Half-life" refers to radioactive decay, "opposing force" refers to Newton's third law, and "blue shift" refers to a shift in light's wavelength towards the blue end of the spectrum due to the Doppler effect. It may also refer to Cherenkov Radiation
 * When Gordon puts on his hazard suit in the first level, there are two empty hazard suit slots. According to Decay developer Gearbox's CEO Randy Pitchford (aka Duvalmagic) these belonged to (or were used by) Decay protagonists Gina and Colette.