S.T.A.L.K.E.R. is a first-person-shooter survival horror video game franchise developed by Ukrainian game developer GSC Game World. The series is set in an alternate version of the present-day Chernobyl Exclusion Zone in Ukraine, where, according to the series' backstory, a mysterious second Chernobyl disaster took place in 2006. As a result, the physical, chemical, and biological processes in the area were altered, spawning numerous nature-defying anomalies, artifacts, and mutants. The player takes the role of a "stalker" - a name given to trespassers and adventurers who have come to explore the exclusion zone and its strange phenomena.

The series is loosely based on the novel Roadside Picnic by Arkady and Boris Strugatsky, and influenced by the 1979 movie Stalker by Andrei Tarkovsky which was itself adapted from "Roadside Picnic".^[1] The name "S.T.A.L.K.E.R." is a backronym for Scavengers, Trespassers, Adventurers, Loners, Killers, Explorers and Robbers.

The franchise takes place in an alternate Chernobyl Exclusion Zone (or simply "the Zone"), where a number of classified laboratories were established following the nuclear disaster of 1986. Due to the experiments conducted in these laboratories, a second catastrophe occurred in 2006, which caused physical and meteorological phenomena all over the exclusion zone and mutation of local plant and animal life.^[2] After the second disaster, the Zone grew rich with anomalies — inexplicable phenomena that don't obey the laws of physics or common sense (such as pillars of fire, lightning scatter, whirlwinds that tear everything that falls into them, etc).^[3]

The anomalies are causing artifacts to appear, which are items with unique and extraordinary properties such as anti-gravity or radioactivity absorption.^[4] People who have come to explore the Zone in search of personal enrichment such as artifacts are known as "stalkers". Many of them are working on their own, though there are also several organized factions present. The Duty are a paramilitary organization that believes the Zone is a threat to humanity that must be destroyed. Conversely, members of the Freedom faction believe the Zone is a gift that should be freely accessible to all people. Bandits are wanted criminals and outlaws that have come to the Zone to hide from authorities, while the Clear Sky seek to understand the origins of the Zone.

To protect the Zone from intruders, a military checkpoint known as the Cordon was established by the Ukrainian Military Forces. The military have an uneasy relationship with stalkers - while official orders are to shoot any trespassers on sight, military personnel are often bribed to look the other way. Additionally, military squads carry out operations in the Zone, such as elimination missions or securing strategic points. The most dangerous of the Zone dwellers are other stalkers, particularly a fanatical sect called the Monolith that protects the center of the Zone, and mutants, some of which possess psionic powers.

The protagonists in the series have different goals and allegiances, but often must work together. Typically, every game's primary goal is to reach the center of the Zone, with a number of adventures, dangers, and challenges on the way.

The S.T.A.L.K.E.R. series has positive reviews from popular gaming sites and was well received by critics. The series is scored between 75 and 82 on Metacritic.

By August 2010, the franchise had sold over 4 million copies.^[23] In August 2021, GSC and Koch Media is claiming over 15 million total sales for the franchise.^[24]

A board game is being developed by Awaken Realms.^[25]

In 2010 the first game of the Metro game series was released. Metro is another franchise of Ukrainian first-person shooter games based on the Metro 2033 literature series, which was created by some ex-members of the S.T.A.L.K.E.R. development team who left to form 4A Games in 2006 before the release of Shadow of Chernobyl.

The former S.T.A.L.K.E.R. 2 team opened a new studio, Vostok Games, in 2012. In 2015, they released a free-to-play massively multiplayer online first-person shooter game titled Survarium in the spirit of the franchise, using ideas they created for the cancelled sequel.^[26] Their new project is a battle royale game set in Chernobyl, titled Fear the Wolves.

In 2014, West-Games, which claimed to be composed of former S.T.A.L.K.E.R. core developers (according to both GSC Game World^[27] and Vostok Games,^[28] falsely) launched a Kickstarter campaign for a spiritual successor to S.T.A.L.K.E.R. first called Areal^[29] and then STALKER Apocalypse. While it managed to reach its goal of $50,000, multiple concerns were raised throughout the campaign about the project being a possible scam, and Kickstarter eventually suspended the campaign two days before its deadline, for undisclosed reasons.^[30][31]

In 2019, Alexey Sityanov, former game designer and story writer of Shadow of Chernobyl, Survarium and Sketch Tales, teamed up with The Farm 51 to work on their Kickstarter project, Chernobylite. The game features similar gameplay and themes to S.T.A.L.K.E.R, and the environment is based on the real Chernobyl exclusion zone, done by utilizing photogrammetry measurements.^[32][33] A stalker is introduced in the game as an antagonist, known as Black Stalker. Chernobylite released the first early access version of the game on 16 October 2019, on Steam.^[34]