Short answer
Space Marines, or the Adeptus Astartes, are humans turned into super-soldiers with implanted organs grown from a Primarch's genes. They stand well over seven feet tall in armor, heal fast and fear almost nothing. Since the Horus Heresy they've been organized into Chapters of about 1,000 warriors. There are roughly a thousand Chapters, each defending its own region of the Imperium.
How a Space Marine is made
Every Space Marine starts as a boy, usually picked from a harsh world where only the toughest survive. Chapters test candidates brutally. Many die during the trials. Those who pass become Neophytes.
Over several years, the recruit is given a series of surgically implanted organs. The standard set has nineteen. They do things like add a second heart, toughen the bones, let him absorb memories by eating flesh and link his nervous system to his armor. The organs are grown from gene-seed, genetic material passed down from the Chapter's Primarch.
Two of the implants, the progenoid glands, collect a copy of the gene-seed as the marine lives. When he dies, his Chapter recovers them to make the next generation. That's why Space Marines will fight hard to retrieve their fallen.
In the lore, Space Marine gene-seed only works on boys, so every Space Marine is male. That rule is specific to Space Marines. In 2024 Games Workshop confirmed that women serve in the Adeptus Custodes, the Emperor's separate bodyguard, which is a different organization with a different process.
Gene-seed carries the Primarch's quirks with it. Blood Angels suffer the Red Thirst and the Black Rage, echoes of their Primarch Sanguinius's death. Space Wolves risk turning into feral, wolf-like beasts. Many Chapters have damaged gene-seed that's lost some organs.
From Legions to Chapters
During the Great Crusade, Space Marines fought in eighteen giant Legions. Some held over a hundred thousand warriors. When the Horus Heresy showed what happened if one Primarch could command that many, the Imperium decided it could never happen again.
Roboute Guilliman wrote the Codex Astartes, a huge manual of tactics and organization. It ordered the loyal Legions split into Chapters of about a thousand. The original Legion kept its name, and the new ones became its successor Chapters. This split is called the Second Founding. More foundings followed over the centuries, which is how the number of Chapters grew to around a thousand.
Not every Chapter follows the Codex closely. The Space Wolves ignore large parts of it. The Black Templars run as a crusading fleet. Some Chapters follow it word for word. How closely a Chapter sticks to the Codex is part of its personality.
How a Chapter is organized
A Codex Chapter has ten companies of about a hundred marines each, plus its command staff and specialists. The companies have set jobs.
| Company | Role | What it does |
|---|---|---|
| 1st | Veteran company | The Chapter's most experienced warriors. Often fights in Terminator armor. |
| 2nd to 5th | Battle companies | The front line. Each mixes different squad types so it can fight alone. |
| 6th and 7th | Battleline reserve | Backs up the battle companies with more line troops. |
| 8th | Close support reserve | Specialists in fast assaults and close fighting. |
| 9th | Fire support reserve | Heavy weapons squads. |
| 10th | Scout company | New recruits learning the trade, plus specialists in stealth and recon. |
Inside each company, marines fight in squads. Most squads hold five or ten warriors under a sergeant. The chain of command runs like this.
| Rank | Leads |
|---|---|
| Chapter Master | Commands the whole Chapter of about 1,000 marines. |
| Captain | Leads one company of about 100. |
| Lieutenant | A captain's second, often leading half a company. |
| Sergeant | Leads a squad, usually five or ten marines. |
| Battle-Brother | A full Space Marine. |
| Neophyte / Scout | A recruit still completing his transformation and training. |
Specialists outside the companies
- Chaplains look after the Chapter's spiritual life and its fighting spirit.
- Librarians are psykers who use their warp powers in battle.
- Apothecaries heal the wounded and recover gene-seed from the dead.
- Techmarines are trained on Mars to maintain the Chapter's machines and armor.
Specialists are attached to companies as needed but belong to their own branches within the Chapter.
Primaris Marines
When Guilliman returned at the end of the 41st millennium, he brought a new kind of Space Marine with him. Primaris Marines are taller and stronger than the old kind, with three extra implanted organs. They were developed in secret over ten thousand years by Archmagos Belisarius Cawl, on orders Guilliman had left before his long sleep.
Huge numbers of Primaris marines reinforced existing Chapters, and many new Chapters were founded from them. Older marines, sometimes called Firstborn, can now undergo a risky procedure called the Rubicon Primaris to become Primaris themselves. By now, Primaris and Firstborn serve side by side in most Chapters, and the difference matters far less than it did when they first arrived.
Famous Chapters
- Ultramarines: Guilliman's own Chapter, and the model Codex Chapter.
- Dark Angels: secretive knights hunting the Fallen, traitors from their own past.
- Blood Angels: noble, artistic warriors struggling against a curse in their blood.
- Space Wolves: rowdy, fierce warriors who follow their own traditions.
- Imperial Fists: stubborn masters of defense and sieges.
- Black Templars: zealous crusaders who believe the Emperor is a god.
The Grey Knights and the Deathwatch are special cases. The Grey Knights are a secret Chapter built to fight daemons. The Deathwatch borrows veterans from many Chapters to hunt aliens. For the wider list of armies, see our factions guide.
Corin Ashdown is the pen name of the editor who writes and checks every guide on Vorbelutrioperbir. How we write and check guides.