Military The Ukrainian military supposedly has the Zone contained in order to prevent the looting of dangerous, radioactive materials and to prevent the unauthorized entering of any individuals. They allegedly enforce this rule with a "Shoot-on-sight" declaration. In an ironic twist, because of major corruption within their ranks, they themselves are the reason why Stalkers can continue to operate, and why radioactive materials from the Zone keep flowing to the outside world. Overtly, the Military are hostile to all Stalkers to keep up appearances, but they also make shady dealings with them at the same time.
Bandits The Bandits are the raiders and marauders of the Zone, ranging from petty thugs and gangsters to organized crime syndicates. Apart from Freedom, which treats them with marginal contempt, everyone hates the Bandits due to their nefarious acts of robbing and exploiting honest Stalkers. Most Bandits are unorganized and leaderless; there are only a few known Bandit leaders and camps, and even then their groups are still fairly unorganized, with underlings always trying to take control by overthrowing the previous leader. Their unit size is mediocre, and their equipment is cheap and shoddy; because of this they've adapted to a guerilla style of warfare, ambushing small groups with large numbers by camping in locations just off the normal paths.
Duty Duty was founded by former military expedition survivors and Stalkers concerned about the looming horrors of the Zone. They are one of the largest Stalker clans in the Zone. They have a strict military code (such as no smoking, maintaining ones weapons, etc...) and operate in a military fashion, using ranks like Sergeant, Major, Captain, etc. Their goal is to contain and destroy the Zone, fearing that it will spread to the outside world. They view the Zone as an ulcer on the world and their efforts are a way to excise it. They do this with blunt force by killing mutants, eliminating all who oppose them or exploit the free, and burning down or drowning mutant dens. They are neutral toward other Zone factions, save for the Bandits and their long-time nemesis: Freedom. It is also said that Duty has deep connections within the military, specifically Spetsnaz detachments within the Zone, although they treat regular grunts with utter contempt.
Freedom Freedom is a collection of anarchists, daredevils, and other Stalkers who believe in the right for free access to the Zone. They are one of the largest Stalker clans in the Zone. Freedom believes that the Zone is a wonder given to all humanity, and feel that it should be further studied in order to fully reap its rich pool of untold information. They openly oppose the Military and their long-time rivals Duty, who hold the polar opposite of their ideals and beliefs. Freedom also studies the Zone, in their own fashion, but not as extensively as the Ecologists. They focus more on documenting the Zones anomalies, mutant behavior, and their movements.
Loners Loners are a nomadic semi-unified group of Stalkers who simply live off the Zone in order to earn some coin for their daily lives. Loners are neutral toward almost all other factions, but do have complications with the Military, and openly oppose the Bandit faction for their actions against free Stalkers. Most Loners are only concerned with the riches of the Zone, and do not pay attention to the other factions hatred for each other - provided, of course, they do not threaten them. Because of this, Loners are often recruited by various factions as mercenaries.
Mercenaries The Mercenaries are a private military force operating within the Zone. They are professional soldiers who will do just about anything in the Zone, ranging from retrieval, assassination, etc, though of course it must be for the right price. Mercenaries are usually neutral toward other factions, as they are a source of income, unless they interfere with their operations.
Ecologists The Ecologists are Ukrainian scientists sent into the Zone on behalf of the government, but seem to be affiliated with outside organizations. Their main purpose is the study of all aspects of the Zone, using scientific methods to further explain what the Zone is, and its purpose. The Ecologists are neutral to all factions, and are not involved in any of their conflicts. Due to the military's slipping grip on the Zone, the Ecologists rely on opportunistic Dutyers, Loners, and Mercenaries to provide them protection, and to perform errands that may require combat, since the scientists are better thinkers than fighters.
Monolith The Monolith were originally ordinary Stalkers who believed in the power of the Wish Granter. Prior to Clear Sky, the faction reportedly made an exodus to the Zone's centre and disappeared. After the large emission during the events of S.T.A.L.K.E.R.: Clear Sky, they became active, venturing back from beyond the Barrier and attacking Stalkers. The Monolith are the front line soldiers of the C-Consciousness, and would rather die than let any Stalker reach the sacred crystal. Devoid of higher reasoning and emotion, they nevertheless carry out orders with fanatical zeal and dedication. The Monolith faction has no allies, and actively attacks all other factions.
Clear Sky Clear Sky is the most secretive faction within the Zone. The faction was disbanded after losing the majority of their manpower during the events in S.T.A.L.K.E.R.: Clear Sky. They are possibly affiliated with outside organizations, indicated by the extensiveness of their research and resources to carry out their operations. Their aims are similar to the Ecologists, but they are more than willing to take up arms against those who oppose them.
International Scientific Group The International Scientific Group are a collaborative research organisation with members from across the world, who have come together to attempt to uncover the secrets of the Chernobyl Exclusion Zone. Made up of scientists, private military contractors and opportunistic Stalkers, the Group aims to reveal the long-kept Soviet secrets buried beneath the Zone.