Warhammer 40,000 (informally known as Warhammer 40K or just 40K) is a tabletop miniature wargame produced by Games Workshop, set in a science fantasy universe. Warhammer 40,000 was created by Rick Priestley in 1987 as the futuristic companion to Warhammer Fantasy Battle, sharing many game mechanics. Expansions for Warhammer 40,000 are released from time to time, often to facilitate a certain sort of game, such as Cities of Deathand Apocalypse, which give rules for urban and large-scale combat, respectively. The game is currently in its fifth edition.

Players assemble and paint individual 28 mm (1.1 in) scale (approximately 1:56) miniature figures that represent futuristic soldiers, creatures and vehicles of war. These figurines are collected to comprise squads in armies that can be pitted against those of other players. Each player brings a roughly equal complement of units to a tabletop battlefield with handmade or purchased terrain. The players then decide upon a scenario, ranging from simple skirmishes to complex battles involving defended objectives and reinforcements. The models are physically moved across the table and the actual distance between models plays a role in the outcome of combat. Play is turn based, with various outcomes determined by tables and the roll of dice. Battles may last anywhere from a half hour to several days, and battles may be strung together to form campaigns. Some game and hobby stores host games periodically, and official tournaments are held on a regular basis.

Warhammer 40,000's gothic space fantasy setting spans a vast fictional universe. Its various factions and races include the Imperium of Man (the human race 38,000 years hence), the Orks(similar to Warhammer Fantasy Orcs), and the Eldar (similar to Elves in Warhammer Fantasy Battle). These races, along with their playing rules, are covered in the game's rule books and supplemental army codexes, along with articles in the Games Workshop magazines, White Dwarf and Imperial Armour. Lines of these miniatures are produced by Citadel Miniatures and Forge World.

The Warhammer 40,000 setting is used for several related tabletop games, video games, and various works of fiction, including licensed works published by Black Library, a subsidiary of Games Workshop.

Each of the Chaos Space Marine Legions fights using a different style of warfare; also, four of the nine are dedicated to one of the four major Chaos Gods. Codex: Chaos Space Marines (Chambers et al., 2002) includes the current rules for fielding a Chaos Space Marine army in a game ofWarhammer 40,000; it also includes rules for fielding the troops of the specific legions that are dedicated to any of the Chaos Gods, with rules for the other legions being removed in the 4th edition Codex.

The nine Chaos Marine Legions are (in order of founding) Emperor's Children, Iron Warriors, Night Lords, World Eaters, Death Guard, Thousand Sons,Black Legion, Word Bearers, Alpha Legion.

The Emperor's Children has a number of notable members, those include Lucius the Eternal, Saul Tarvitz, Fabius Bile, Gaius Caphen, and Charmosian. The legions are often featured in short stories and novels. For example, The Night Lords appeared in the short story Chains of Command and also "Lord of the Night", a story that follows the Raptor lord Zso Sahaal in his struggle to regain his dead Primarchs stolen legacy, adding an entirely different view and depth to a legion that had previously little attention. Another appearance is made in the Space Wolves story "Sons of Fenris", where a group of Night Lords are fighting against Wolf Lord Ragnar Blackmane and his cadre of Wolfguard. They also appeared in the Eldar stronghold mission ofWarhammer 40,000: Dawn of War: Dark Crusade (it is said that they were lured by the Eldar as a distraction). The Iron Warriors have appeared in the novel "storm of Iron" and favour siege warfare.

In addition, since the Horus Heresy, several Space Marine Chapters have been corrupted in one way or another by Chaos. Very little background information has been provided about these Chapters, and no specific rules exist for them, allowing players to adapt other rules to represent their forces.

According to the records of Ordo Malleus approximately 50 chapters have turned renegade after the Horus Heresy, however the accuracy of this number is doubtful. Examples of given space marine chapters include the Sons of Malice, the Damned Company of Lord Caustos, the Violators, the Steel Cobras, the Thunder Barons, and the Astral Claws (known as the Red Corsairs since their rebellion). Their numbers are even harder to gauge, as usually when a chapter succumbs to Chaos there will be marines who remain loyal to the Emperor. One example is the Battle-Captain of the Death Guard's 7th Company, Nathaniel Garro, who rallied his company and escaped with other surviving loyalists from Isstavan III, returning to Terra and warning them of Horus's betrayal. Another example is Iacton Qruze, the only member of the Sons of Horus to remain loyal to the Emperor and survive the Horus Heresy (both of these marines are rumored to have formed the core of what would become the Imperial Inquisition). However, these loyalist survivors are rare, as they are usually outnumbered and slaughtered by their corrupted brethren.