Short answer
The Imperium of Man is the human empire of Warhammer 40k, spread across roughly a million worlds. In name, the Emperor rules it from the Golden Throne. In practice, it was run for ten thousand years by a council called the High Lords of Terra, and since his return it has been led by Roboute Guilliman as Lord Commander and Imperial Regent. Under them sit dozens of huge, rival institutions that do the actual governing.
How it came to be
The Emperor built the Imperium during the Great Crusade, uniting human worlds that had been cut off from each other for thousands of years. He meant it to be a secular empire of reason. He banned religion and called his philosophy the Imperial Truth.
The Horus Heresy ruined that plan. After the Emperor was placed on the Golden Throne, he could no longer lead. The people who took over turned him into a god. Over the centuries that followed, the worship of the Emperor, the Imperial Creed, became the state religion. The church that grew around it became one of the most powerful forces in the Imperium.
For the next ten thousand years the Imperium mostly stood still. It couldn't invent new technology, only copy old designs. It grew suspicious of everything. It survived through sheer size and stubbornness.
Who's actually in charge
The High Lords of Terra are a council of twelve senior officials, usually the heads of the biggest institutions. For most of the Imperium's history, they made the real decisions. Their council was famous for infighting, slow decisions and occasional assassinations. Which offices hold a seat has changed over time, and sources list slightly different members.
That changed when Roboute Guilliman returned. He took the title Lord Commander of the Imperium and was named Imperial Regent, which put him above the High Lords. He launched a huge military campaign called the Indomitus Crusade, began writing a new civic codex to reorganize how the Imperium is governed and has been trying to fix it ever since. He's not the Emperor, though, and plenty of officials still resent him.
Two forces sit mostly outside the normal chain. The Adeptus Mechanicus of Mars is a partner more than a servant, with its own leaders and its own religion. The Inquisition answers to no one but itself. Any single Inquisitor can command armies or condemn a planet.
The Imperium org chart
Here's the structure laid out from the top down. It's simplified, because the real thing has thousands of sub-departments, but these are the bodies you'll meet in the lore.
| Tier | Body | What it does | Answers to |
|---|---|---|---|
| Top | The Emperor | Nominal ruler. A god to the faithful. Says nothing. | No one |
| Top | Lord Commander and Imperial Regent | Roboute Guilliman, who runs the Imperium in the Emperor's name. | The Emperor |
| Top | High Lords of Terra | The ruling council of the heads of the great institutions. | The Lord Commander |
| Civil | Administratum | The bureaucracy. Collects tithes, keeps records, runs everything slowly. | High Lords |
| Civil | Adeptus Astra Telepathica | Recruits and trains psykers, including the astropaths who send messages across space. | High Lords |
| Civil | Adeptus Astronomican | Runs the psychic beacon on Terra that ships navigate by. | High Lords |
| Faith | Adeptus Ministorum | The Ecclesiarchy, the church of the Emperor. Its military arm is the Adepta Sororitas. | High Lords |
| Tech | Adeptus Mechanicus | The tech priests of Mars. Build and maintain the Imperium's machines. | Allied, own leadership |
| Military | Astra Militarum | The Imperial Guard. Billions of ordinary soldiers. | High Lords |
| Military | Imperial Navy | The warships that move and protect everything else. | High Lords |
| Military | Adeptus Astartes | Space Marine Chapters. Largely independent. | The Lord Commander, in practice |
| Military | Adeptus Custodes | The Emperor's bodyguard on Terra. | The Emperor alone |
| Shadow | Inquisition | Hunts heretics, aliens and daemons with almost unlimited authority. | Nobody, in practice |
| Shadow | Officio Assassinorum | The Imperium's assassins. | High Lords |
| Law | Adeptus Arbites | Enforces Imperial law on planets, but not local law. | High Lords |
| Local | Planetary governors | Rule individual worlds however they like, as long as tithes are paid. | The Administratum |
Life in the Imperium
For most citizens, life is hard. Huge hive worlds pack billions of people into towering cities. Factory worlds produce weapons. Agri worlds grow food for planets that can't feed themselves. Every world owes the Imperium a tithe of goods, soldiers or psykers.
Most people never leave the planet they were born on. They may never see a Space Marine. They pray to the Emperor, work long shifts and hope not to be noticed by the Inquisition. Planetary governors can rule however they like, as long as the tithe gets paid and heresy is stamped out.
Communication is slow and unreliable. Messages between stars are sent by astropaths, psykers who shout them through the warp as dream-like images. They can arrive garbled, late or not at all. A world might wait years for help, or keep fighting a war that ended decades ago because nobody told it. This is a big reason the Imperium feels so disjointed. Its parts often don't know what the other parts are doing.
Faith holds it together where logistics can't. Almost every citizen, from a governor to a farm laborer, shares the belief that the Emperor protects. That shared creed is the closest thing the Imperium has to glue, and its preachers know it.
Psykers have it worst. Untrained psykers can act as doorways for daemons, so they're hunted down. Many are shipped to Terra. Some are trained to serve. Others are fed to the Golden Throne to keep the Emperor alive.
The Imperium today
At the end of the 41st millennium, Abaddon's Thirteenth Black Crusade destroyed the fortress world of Cadia. A giant warp rift opened across the galaxy, known as the Great Rift or the Cicatrix Maledictum. It split the Imperium in two.
The half on Terra's side is called Imperium Sanctus. The half beyond the Rift is Imperium Nihilus, cut off from the light of the Astronomican, where many worlds are fending for themselves. Guilliman's crusade has pushed back in places, and new Space Marines have been sent to many fronts. But the Imperium faces more threats than ever, from Chaos, from waking Necrons and from Tyranid swarms. The story is still being told.
Corin Ashdown is the pen name of the editor who writes and checks every guide on Vorbelutrioperbir. How we write and check guides.