The '''Codex Astartes''' is the doctrine of the [[Space Marine Chapter]]s, governing all aspects of Chapter organisation and battlefield tactics. For any given tactical situation, the Codex has hundreds of pages devoted to how it may be met and overcome. The wisdom of thousands of Imperial warriors has contributed to the Codex, and details on everything from unit markings to launching a full-scale planetary assault are contained within its pages.{{CMEFn|25b}}

In the aftermath of the Horus Heresy, the [[Emperor]] had been incapacitated, and the Space Marines' numbers had been decimated by defections to [[Chaos]] and battle losses. With the Emperor confined to the [[Golden Throne]] the [[High Lords of Terra]] found themselves the new leaders of the Emperor's domain. Both the High Lords and surviving Primarchs became paranoid of the possibility of such events repeating themselves. The armies of the Emperor once again scoured the galaxy seeking out all forms of heresy and removing even the slightest chance of rebellion. The Ultramarines were left as the largest loyal legion, and Guilliman assumed the title of [[Lord Commander of the Imperium]].{{CMEFn|25}} Concerned that once again their could be a threat from such a terrible force each Legion was to be divided into smaller Chapters and a new set of regulations would be created to govern them.{{Fn|9}}

The Codex Astartes was a code developed by Guilliman. This tome would contain everything a member of the Astartes would need to know to function. From recruitment to organization to tactics and beyond the Codex Astartes became a sacred text to the majority of the Chapters.{{Fn|10}} One of the most significant changes under the new Codex was the newly founded Chapters. Where the Legions could number more than 10,000 strong the Chapters would be limited to about a thousand. Each original Legion would be divided as many times as needed into successor Chapters. This division would come to be called the [[Second Founding]] seven years after the Horus Heresy{{Fn|11b}}, in 021.[[M31]]. The remaining thousand bodies in the original Legion would also survive as a new Chapter of the same name. While these newly created Chapters claimed their founders origins, such as their Primarch, they would also come to develop their own culture and customs.{{CMEFn|25a}}

[[Image:CodexOrganisation.jpg|right|300px|thumb|Pre-[[Primaris Space Marine]] Codex Astartes Space Marine Chapter organisational chart.]]{{CME}}

Likewise, the [[Dark Angels]] and their successors, while dividing into chapters, have remained united in their single-minded pursuit of the [[Fallen]].{{CMEFn|26}}

This means the number of squads in a perfectly compliant chapter are now 44 ''[[Battleline]]'', 18 ''[[Close Support]]'', and 18 ''[[Fire Support Squad|Fire Support]]'', in addition to 10 squads of veterans and 10 squads of scouts and [[Vanguard Space Marine]]s, spread across the chapter's companies as detailed below.{{CMEFn|15a}}{{Fn|15b}}

Each Chapter has its own unique colour scheme and Chapter Badge; however, Codex Chapters all follow a common heraldry system, though there are many slight variations from the system below.{{CMEFn|15b}}

Each Company has a unique colour that its members wear, commonly on the shoulder plate rims{{Fn|15b}}, but some chapters use chest eagles, bolter cases, knee pads, helmets, or other parts of a Space Marine's [[Power Armour]]:{{CMEFn|15b}}:

* 2nd Company: Yellow {{Color sample|size=15|yellow}}{{CMEFn|15b}} or Gold {{Color sample|Gold|Yellowsize=15|}}{{Fn|15b}}

* 10th Company: Company colour is not displayed on their armour, because of their need to blend with their surroundings on reconnaissance missions.{{Fn|27a}}

The left shoulder plate of a Space Marine in Power Armour always shows the Marine's Chapter Badge. The right pad shows the squad badge, this shows the type of squad the marine belongs to and the number of his squad. [[Veteran Squad]]s use a stylised version of the [[Crux Terminatus]] similar to a Cross Pattée, Tactical Squads use an arrow, Assault Squads use four perpendicular arrows pointing outwards and Devastator Squads use an inverted V. Space Marines in Terminator Armour wear the [[Crux Terminatus]] on their left pad, and their Chapter Badge on their right pad.{{CMEFn|27a}}

*[[Space Marine Captain|Captains]] wear the heraldry of their company, sometimes embellished with their own personal heraldry{{CMEFn|27d}}*[[Librarian]]s wear blue armour, irrespective of what chapter they belong to, and wear their Chapter Badge as normal. (This practice is now widely ignored, even among the more standard chapters.){{CMEFn|27f}}*[[Chaplain]]s wear black armour, irrespective of what chapter they belong to, and wear their Chapter Badge as normal.{{CMEFn|27e}}*[[Techmarine]]s wear red armour, irrespective of what chapter they belong to, and wear their Chapter Badge as normal.{{CMEFn|27h}}*[[Apothecaries]] wear white armour, irrespective of what chapter they belong to, and wear their Chapter Badge as normal.{{CMEFn|27g}}*[[Space Marine Sergeant|Sergeant]]s wear the same heraldry as their squad, but carry the squad's banner, which displays the Chapter Badge and the squad's number, plus a red skull (the symbol of a sergeant). Sergeants often paint their helmets red, or in some cases wear a red skull on their helmet instead.{{CMEFn|27a}}*[[Space Marine Veteran Sergeant|Veteran Sergeant]]s wear the same heraldry as their squad, but carry the squad's banner, which displays the Chapter Badge and the squad's number, plus a red skull (the symbol of a sergeant). Veteran Sergeants often paint their helmets red and add an additional white stripe or laurel wreath.{{CMEFn|27c}}*[[Terminator]]s: veteran battle-brothers who make use of the [[Tactical Dreadnought Armour]].{{CMEFn|27b}}