A Space Marine Chapter is a completely autonomous military unit made up of the superhuman warriors known as Space Marines or Astartes and their related vehicles, starships and support personnel. Each Chapter serves collectively with the others of its kind as part of the Imperium of Man's Adeptus Astartes. Every Chapter is entirely autonomous from every other Adepta of the Imperium, including the Inquisition, and takes its orders only from the High Lords of Terra themselves. Most Chapters have feudal title to an entire planet of the Imperium which serves as its Chapter homeworld or to a large fleet of starships that serves as a mobile headquarters if the Chapter is fleet-based. The majority of Space Marine Chapters follow the Codex Astartes, a sacred tome written by the Ultramarine Primarch Roboute Guilliman. The Codex Astartes details the proper organisational structure and mobilization methods of a Space Marine Chapter. The Codex has been adopted by the vast majority of Space Marine Chapters as their organisational blueprint. Consequently, most Space Marine Chapters are considered "Codex Chapters" or "Codex-compliant Chapters" due to their adherence to the Codex.
Notable Chapters that do not follow the Codex Astartes include the:
The creation of a new Chapter of the Adeptus Astartes is known as a Founding, and it does not happen overnight. The process is carried out by the Adeptus Mechanicus, which by ancient custom has the right to expect a tithe of 5% of the gene-seed of every Space Marine Chapter to assist it in the creation of new Chapters and to check that the purity of the existing Astartes Chapters' DNA has remained untainted by mutation or exposure to Chaos. Each Chapter is created from the gene-seed of an existing donor Chapter. The zygote derived from each type of gene-seed is implanted by the Magi Biologis of the Adeptus Mechanicus in a human test-slave who spends his entire life in a static experimental capsule, immobile and serving as nothing but a medium from which two progenoid glands will develop. When the progenoids are developed, they are extracted from the original test-slave and then implanted into another two test-slaves, producing four progenoids, and so on. It takes 55 years of reproduction to create a healthy set of 1,000 new Astartes organs. These must be sanctioned officially by the Fabricator-General of the Adeptus Mechanicus and then by the High Lords of Terra, speaking for the Emperor of Mankind, who alone can give permission for the creation of a new Chapter.
Each Codex-compliant Space Marine Chapter consists of 1000 fighting Space Marines plus support staff which are divided into 10 companies. Each Chapter is led by a Chapter Master. Unlike the Imperial Guard, the Adeptus Astartes has no leadership above the Masters of the individual Chapters. Also unlike many other Imperial military forces, the Astartes has no members to represent the organization as a High Lord of Terra on the Senatorum Imperialis. Besides the Space Marine fighting brethren, there are many non-combat personnel, fully part of the Chapter but generally not involved directly in combat. These include the Chapter's hereditary slaves or bonded serfs, Astropaths, Navigators, etc. Each company of a Codex-compliant Chapter has its own command staff, consisting of a:
Most companies will usually be accompanied by a Command Squad of five Veteran Space Marines that will include:
- Company Captain
- Company Champion
- Apothecary - For a Space Marine Chapter to have a future it must carefully protect and preserve the organic implants created from gene-seed that turn an initiate into a full-fledged Space Marine. An Apothecary serves as a Chapter medic and the overseer of the Chapter's gene-seed and implantation process for the creation of new Space Marines.
- Standard Bearer - Standard Bearers carry the company banner that shows the company's feats and prowess. Captains may also display their personal heraldry in the banner. On rare occasions, the Chapter Banner might be fielded by a particular company's Command Squad.
The First Company of any Codex-compliant Space Marine Chapter is made up of Veteran Marines. These skilled warriors are the only members of the Chapter trained in the use of Terminator Armour. The Second to Fifth Companies of the Chapter are referred to as Battle Companies. Each Battle Company consists of:
- six 10-man Tactical Squads
- two 10-man Assault Squads
- two 10-man Devastators Squads
- Support tanks, bikes, land speeders and Rhino armoured transports
- Dreadnoughts - Dreadnoughts are revered warriors who are mortally wounded and entombed in a cybernetic Dreadnought sarcophagus which serves as a heavy support combat walker
The Sixth to Ninth Companies of a Space Marine Chapter are its Reserve Companies. The Tenth Company of a Space Marine Chapter is the Scout Company composed of new Space Marines who are being exposed to combat for the first time. Those Scout Marines who survive are welcomed to the Chapter as full Brother Marines and are given a suit of Power Armour and moved into one of the Reserve Companies to begin climbing the Chapter's hierarchy--if they survive.
- Codex Imperialis
- Index Astartes I