The local animals and plants were altered a lot by the years of radiation. They have become more dangerous, and have developed natural defenses to survive the harsh world. There are also a lot of people that were mutated by the radiation from the second explosion in the power plant.
The stalkers find it difficult to hunt for artifacts because the Ukrainian Military is in the area, trying to stop the zone getting bigger, and help control the mutant. The Military is an enemy of anyone who is not Military, and will work as a big team to attack anyone they see using heavy force and the use of Mi-35 Hind gunships, which are attack helicopters.
A few other groups work in the zone, and are neutral or an enemy based on certain events. Two groups are important: "Freedom" and "Duty" as they are the only two groups that the player can decide if they want to be friendly towards or not. Neither Faction will attack the player at first, and both will offer missions to the player. Freedom and Duty are at war with one another, and although th player can keep friendship with both of them at first, the player will end up having to choose a side. The player cannot join either faction, no matter how friendly the player becomes towards them, as the player's faction status in the player's P.D.A. will tell the player, the player is always a loner in this game. There are several other groups, these are almost all enemies for the player. The scientists are different though, because the player can be friends by saving one of them from a mercenary attack later in the game.
The player also works with other NPCs in the Zone, and can help Stalker factions and receive missions, which further the storyline. Players are able to explore over 30 square km of the Zone, including the ghost town of Pripyat and the empty Chernobyl Nuclear Plant, while meeting animals and plants mutated by radiation as well as other Stalkers and NPCs.
The game uses uses GSC Gameworld's proprietary ALife artificial intelligence engine. ALife supports more than one thousand characters living in the Zone.
The NPCs act as if they really are alive, having different states, such as task accomplishment, combat, rest, feeding and sleep. The monsters in the game also act in this way. These monsters will move in large groups. The non-scripted way the characters work means that there are an unlimited number of random quests. For example, rescuing Stalkers from danger, killing Stalker renegades, protecting or attacking Stalker camps or searching for treasure. The AI characters travel around the entire zone as they want.
Numerous different ways of playing the game can be used, such as using stealth and sniping. The NPCs will react in a different way to each of them.
A weather system works in various parts of the landscape and let the weather act as if it is real, letting the game have weather such as sunshine, storms and showers. The weapons available, the way the AI acts, game tactics and ranking systems will depend on the weather.
Multiplayer, like many other new games, will work over both LAN and the Internet with up to 32 players. Currently the three game modes are Deathmatch, Team Deathmatch and Artifact hunt. The game will also use a ranking system.
Early in the development of the game it was named "Oblivion Lost", and the game was planned to have more of a futuristic theme before the idea was reworked to set the game in Chernobyl, being a nearby location to the game's Kiev based development team. Eventually, this was reflected in the change of the game's subtitle to "Shadow of Chernobyl". Screenshots and trailers of this early version can be found on various web sites, depicting the robots and spacecraft originally planned for the game.
While the source of the Zone may still turn out to have extraterrestrial connections (like in "Roadside Picnic", where the Zone is thought to have been an absent-minded, accidental creation of a careless, vastly superior alien race), there is no indication that such more direct manifestations will return.
The game was first announced in November 2001 and has had its release date, originally in 2003, pushed back several times. Meanwhile, hundreds of screenshots of the game have been released, as well as dozen preview video clips, accompanied by other forms of promotion by GSC, such as inviting fans to their offices in Kiev to play the current build of the game. However, due to the delays some considered "S.T.A.L.K.E.R." to be vaporware, like the game "Duke Nukem Forever", or fear that the game may become a new "Daikatana".
In their E3 Awards for 2005, IGN gave "S.T.A.L.K.E.R."<nowiki>'</nowiki>s absence the award of "Biggest PC Surprise"; the runners-up were the absence of "Fallout 3" from the show, and the lack of a new announcement from Blizzard Entertainment.
In February 2005, THQ expressed desire to see the game released toward the end of its 2006 fiscal year (March 31 2006) but maintained that no release date had been set. In October, 2005, THQ confirmed that "S.T.A.L.K.E.R." will not be out "until the second half of THQ's 2007 fiscal year - October 2006 at the earliest." In February of 2006, THQ revised this possible release window, saying the game would not be in stores until the first quarter of 2007.
In an interview at the Russian Gameland Awards, PR Manager Oleg Yavorsky indicated that release was planned for September 2006.
In 2006, the game came 9th in "Wired"'s Vaporware '06 award.
THQ ran a competition in January 2007 offering the lucky winners the chance to play the beta version of S.T.A.L.K.E.R, in a 24-hour marathon session. The event, scheduled to take place on the January 24 2007, was subsequently changed to a 12-hour session days before it was supposed to occur.
On the morning of the event, the 'lucky' winners were met at the venue by the THQ staff that had organized the event, who were embarrassed to report that they had been unable to get any copies of the game. In late February GSC managed to release a public beta. Multiplayer demo was released to public on March 15 2007.
On March 2 2007, it was announced that the game had gone gold.
So far, publications which have reviewed the title have been largely positive, noting the immersive, atmospheric setting and open-ended, rewarding gameplay. Some aspects of the game, such as the translation of printed text, the absence of subtitles for spoken Russian, and general game performance, have received negative criticism. It has also been noted that there are numerous sporadic bugs which can affect gameplay, although most of these are considered minor. However, in the newly released patch 1.0001 it is claimed that many bugs and glitches were fixed. Another common criticism is the fact that less than half of the map is actually accessible by the player.
Publications which have submitted reviews include IGN U.S. (8.2/10), IGN Australia (8.9/10), GameSpot (8.5/10), Australian PC User 95% and "PC Gamer" (87%). Ukraine's own "Gameplay" magazine awarded it 4.5 out of 5. UK based website Mansized scored the game 4/5.
Several years after the Chernobyl disaster, the Zone was occupied and "repurposed" by several research projects run by the Soviet government. Facilities were constructed or expanded under the abandoned military bases and civilian factories and industrial sites in the area, and a large lab was established directly beneath Reactor 4 and its Sarcophagus. These experiments were focused primarily on various aspects of the mind, including creating and enhancing ESP in humans, psychotronic weapons, and an experiment intended to create a human hive mind: the "Collective Consciousness," or "C-Consciousness," project.