The Codex Astartes is the doctrine of the Space Marine Chapters, 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.
The Codex was written and formalised after the Horus Heresy by the Primarch of the Ultramarines, Roboute Guilliman. Some Chapters, such as the Ultramarines and their successors, regard the Codex as a holy text, and follow it without question. Many other chapters also follow the Codex, though not always as strictly as the Ultramarines; these two types of chapters are collectively known as Codex Chapters. Among the Ultramarines, each battle brother must memorise whole sections of the codex, so that within a company there exists an entire record of the text.
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.[Needs Citation] 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.
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. 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, 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.[Needs Citation]Rogal Dorn initially rejected the Codex Astartes and enmity developed between him and Guilliman. Dorn called Guilliman a coward, citing his lack of participation in the defense of the Imperial Palace. Guilliman accused Dorn of being a traitor for refusing the Codex. This enmity quickly involved other Space Marine Legions and a rift developed, Leman Russ of the Space Wolves stood by the Imperial Fists, while Jaghatai Khan of the White Scars and Corax of the Raven Guard supported the Ultramarines. A second civil war appeared likely when the Imperial Fists strike cruiser Terrible Angel was fired upon by the Imperial Navy in connection with Codex crisis.
However, Dorn ultimately relented after spending seven days meditating in the pain glove. There, he concluded that the Legion could no longer serve the Emperor who had been and must serve the Emperor who was, which involved accepting the new order of which the Codex was a part.
After the resurrection of Guilliman in late M41, he began to see many of the flaws that came from overly-strict adherence to the Codex Astartes. As a result, he has reformed much of the Codex and eliminated much of its restrictions on Space Marines. New additions include the addition of Primaris Space Marine formations such as Intercessor, Aggressor, Inceptor, and Reiver Squads, as well as the inclusion of two Space Marine Lieutenants below the Company Captain, each commanding a demi-Company. Furthermore, Squads in the Codex Astartes are now defined as Battleline Squads, Close Support, or Fire Support. These formations exist alongside Veteran Squads, Scout Squads, and Vanguard Space Marine Squads.
This means the number of squads in a perfectly compliant chapter are now 44 Battleline, 18 Close Support, and 18 Fire Support, in addition to 10 squads of veterans and 10 squads of scouts and Vanguard Space Marines, spread across the chapter's companies as detailed below.
Secretly, however, Dorn formulated his own protocol to possibly circumvent the Codex Astartes known as the Last Wall, a code phrase that would require the Imperial Fists and the Successor Chapters to bind together under one supreme commander.
The Space Wolves Legion, however, never fully accepted the new doctrine; rather they held sacred the teachings of their Primarch, Leman Russ. For that reason, the Wolves consist of twelve Great Companies, rather than the ten mandated by the Codex, and their system of organization and promotion bears little resemblance to that of hardline Codex chapters, such as the Ultramarines. Although the Space Wolves attempted to create Successor Chapters, the genetic instability of their gene-seed made a disaster of their first (and possibly last) attempt, the ill-fated Wolf Brothers.
In the immediate aftermath of the Heresy, Nassir Amit, the founder and first chapter master of the Flesh Tearers, expressed little but scorn for the Codex, finding it amusing that Guilliman, who had been absent from the Battle of Terra, should think himself fit to impose restrictions on the other Legions.
The Codex states that a Space Marine Chapter should be split into 10 companies of 100 Marines each, plus a Space Marine Captain, Apothecary, Company Standard and Chaplain for each one.[Needs Citation] In the reforms by Roboute Guilliman after his return to the Imperium at the end of M41, two Lieutenants serve directly below the Company Captain, commanding two demi-companies.
Existing outside the Company level organisation, each chapter has an Armoury consisting of the chapter's Techmarines and tanks, a Librarium consisting of the Chapter's Librarians, a Chapter Space Marine Fleet, and the Chapter Master, plus various headquarters staff (including any Honour Guard and the Chapter Champion), and the Chapter's servitors and human serfs.[Needs Citation] This means that even the most Codex-compliant chapter can have as many Techmarines, Librarians, and vehicle staff (pilots, gunners, etc - this includes Dreadnoughts) as it can muster.
The 1st Company is the most powerful company, consisting entirely of Veteran Space Marines. In addition to having access to the Chapter's rarest and most advanced technology, the 1st Company is the only one trained in and equipped with Terminator Armour. The veterans of the 1st are attached to other Space Marine strike forces rather than the entire formation fighting as one. When serving with other formations they serve as peerless mentors and heroes of legend as well as the most experienced shock troops.[20a]
The 2nd, 3rd, 4th and 5th Companies are known as Battle Companies, as they generally form the main battle force deployed for engagements. These are highly flexible formations that are the principle fighting forces of the Chapter. Each Company is led by a Captain with two supporting Lieutenants. Support staff includes a Chaplain, Apothecary, Company Champion, and Company Ancient. Company Veterans support the senior staff and consist of the most experienced and peerless warriors of the Company. In times of war the Battle Company may also be accompanied by one or more Librarians and and Techmarines. In addition to their command duties, each Company Captain also has a number of titles and duties. Examples include the 2nd Company Captain holding the title of the Master of the Watch, the 4th as the Master of the Fleet, and the 5th as the Master of the Marches.[20a]
Each Battle Company principally has ten squads, six Battleline, two Close Support, and two Fire Support. The Codex, however, makes provisions for there to be twenty squads, for when squads from the reserve companies are attached. Additionally a Company's squads can be split in two, forming combat squads. Each Company is in turn split into two demi-Companies, each led by a Lieutenant. The Companies go to war with their own arsenal of wargear and specialist equipment including Phobos Armour, Gravis Armour, Centurion Warsuits, Invictor Warsuits, Land Speeders, Rhinos, Impulsors, Razorbacks, Dreadnoughts, and Bikes. An entire company can fight in stealth if there is enough Phobos Armour to equip them all. The Company Captain can request additional vehicle assets from the armoury in the form of Battle Tanks and aircraft.[20a]
Even the squads themselves can be broken down into a variety of roles should their Captain require it. Should 3 brothers be detached from their fire support squad to form an Eliminator Squad, the remaining seven can be formed into a Hellblaster Squad, pilot Invictor Warsuits, or fulfil a number of other roles including operating vehicles. Should the strength of a Battle Company be deemed insufficient, he will request support from one of the Reserve Companies, the formation of which depends on the mission. For instance, if he expects he will need fast-moving forces to rapidly secure ground or counter agile enemies, he will request Close Support Squads.
The 6th, 7th, 8th and 9th Companies are the Reserve Companies, designed as training and reserve formations, used to bolster the Battle Companies in combat when needed.
Rather than formations of mixed squad types, Reserve Companies contain squads of the same strategic disposition. The 6th and 7th Companies each maintain 10 Battleline Squads and the 8th and 9th are made up of either Close Support or Fire Support Squads respectively. The Reserve Companies also embed additional specialities to provide the Chapter with the resources to fight campaigns. The 6th and 7th Companies undergo extensive training in vehicle piloting, the 8th specialize in storming defenses, and the 9th excel in providing devastating salvos of fire. Typically squads within the Reserve Companies will fight alongside a Battle Company, though on some occasion - particularly in a larger campaign - they might fight as a whole. Each Reserve Company has a leadership similar to a Battle Company, with a Captain, two Lieutenants, Chaplain, Apothecary, Ancient, and Company Veterans. Just as with the Battle Companies, the Captain of each Reserve Company also normally has an additional title and duties.[20a]
After graduating from the 10th Company, the first destination for a new Space Marine is the Reserve Companies. In the 9th they hone their understanding of wider strategy in the application of heavy firepower, in the 8th they learn the value of rapid assault and the importance of constant movement, while in the 6th and 7th they prove to their brothers and commanders that have assimilated these teachings. After mastering their aspects of war and have demonstrated enough worth will they be promoted to the Battle Companies.[20a]
The 10th Company, also known as the Scout Company, is the first destination for any new Neophyte of the Chapter once they have moved passed the Aspirant stage. There are no regulations dictated in the Codex as to the number of Neophytes a Chapter may have and the recruitment rate is not fixed and methods differ between Chapters. The Neophytes of the 10th have yet to earn the Black Carapace, the final implant that allows them to operate their Power Armour efficiently. Officers of the 10th all closely observe the members of the Company and teach them the way of the Bolter and blade. They also learn how to pilot Bikes and Land Speeders as well as concepts such as tactical withdrawals, target prioritizing, stealth, reconnaissance, and sabotage. Using these lessons they form the intelligence branch of the Chapter that undertake a variety of missions such as picking off sentries, cutting down enemy patrols, supply disruption, and battlefield reconnaissance. In war, they undertake these hazardous missions to measure their strength and ascertain who truly has the skill required to be a true Space Marine. Much like those of the Reserve Companies, members of the 10th are usually assigned to other Companies in war and rarely go to battle as a whole unit.[20a]
Currently, the Scout Company wields a variety of formations besides the basic Scout Squads and Scout Bike formations. It also fields a force of 100 Vanguard Space Marines that may take to the field as Infiltrators, Incursors, Suppressors, Eliminators, and Reivers as well as piloting Invictor Warsuits. Unlike the Scouts, these Vanguard Space Marines are a standing and permanent force within the Company and represent the elite special operations arm of the Chapter.[20a]
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.[Needs Citation]
Each Company has a unique colour that its members wear, commonly on the shoulder plate rims, but some chapters use chest eagles, bolter cases, knee pads, helmets, or other parts of a Space Marine's Power Armour[Needs Citation]:
- 1st Company: White or Silver (members of the 1st Company should also paint their helmet the company colour).
- 2nd Company: Yellow or Gold
- 3rd Company: Red
- 4th Company: Green
- 5th Company: Black
- 6th Company: Orange
- 7th Company: Purple
- 8th Company: Grey
- 9th Company: Blue
- 10th Company: Nominally white; however, the company colour is not displayed on their armour, because of their need to blend with their surroundings on reconnaissance missions.
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 Squads 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.[Needs Citation]
Officers and Specialists
- Captains wear the heraldry of their company, sometimes embellished with their own personal heraldry
- Librarians 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.)
- Chaplains wear black armour, irrespective of what chapter they belong to, and wear their Chapter Badge as normal.
- Techmarines wear red armour, irrespective of what chapter they belong to, and wear their Chapter Badge as normal.
- Apothecaries wear white armour, irrespective of what chapter they belong to, and wear their Chapter Badge as normal.
- Sergeants 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.
- Veteran Sergeants 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.
- Terminators: veteran battle-brothers who make use of the Tactical Dreadnought Armour.
- Vanguard Space Marines: Reconnaissance, infiltration, and assassination specialists.
Known passages of the Codex Astartes:
- "For our enemies will bring us to battle on the caprice of chance. The alien and the renegade are the vagaries of the galaxy incarnate. What can we truly know or would want to of their ways or motivations? They are to us as the rabid wolf at the closed door that knows not even its own mind. Be that door. Be the simplicity of the steadfast and unchanging: the barrier between what is known and the unknowable. Let the Imperium of Man realise its manifold destiny within while without its mindless foes dash themselves against the constancy of our adamantium. In such uniformity of practice and purpose lies the perpetuity of mankind." - Codicil CC-LXXX-IV.ii: The Coda of Balthus Dardanus, 17th Lord of Macragge, entitled Staunch Supremacies 
- "Gather your wits, as the traveller gauges the depth of the river crossing with the fallen branch, before wading into waters wary." - Codicil MX-VII-IX.i: The Wisdoms of Hera
- "If you know both yourself and your enemy, you can win a hundred battles without a single loss. The clever combatant imposes his will on the enemy, but does not allow the enemy's will to be imposed on him."
- "With combat knife, boltgun, and grenade, the Space Marine shall assail his foe. The chainsword is the will of the Space Marine made manifest. The Space Marine shall master all weapons, and all battlefields. When harried, the Space Marine shall drive his enemies back. The Space Marine will never know defeat." 
- "The Tactical Squad shall draw the enemy's fire, thus allowing the Devastator Squad to attack from a position of strength." 
- 1: Games Workshop Website - Ultramarines Background
- 2: The Long Games at Carcharias (Short Story)
- 3: The Chapter's Due (Novel), Chapter Six, pg. 78
- 4: Warhammer 40,000: Dawn of War II - Chaos Rising
- 5: Index Astartes I, Codex Astartes, pgs. 12-16
- 6: Codex: Space Marines (5th Edition), pg. 16
- 7: Codex: Black Templars (4th Edition), pg. 6
- 8: Index Astartes II: Imperial Fists, pgs. 14-15
- 9: Codex: Space Wolves (5th Edition), pgs. 8-9
- 10: Grey Hunter (Novel), [Needs Citation]
- 11: Deathwatch: Rites of Battle, pg. 53
- 12: The Emperor Expects (Novel), Chapter 14
- 13: Flesh of Cretacia (Novella), Chapter 1
- 14: Dark Imperium (Novel), Chapter 10
- 15: Codex: Space Marines (8th Edition), pgs. 16-17
- 16: Codex: Vanguard Space Marines (8th Edition), pg. 5
- 17: Apocalypse Mega-battle and Warhammer 40,000 Preview! (Last Accessed July 3rd 2019)
- 18: Codex: Space Marines (8th Edition, 2nd Codex), pgs. 16-17
- 19: Warhammer Community: Warhammer 40,000 Preview – What’s in the Box? (Posted on 13/06/2020) (Last accessed on 14 June 2020)
- 20: Codex: Space Marines (9th Edition) pg.16
- 20a: pg.20-