Administratum

The Adeptus Administratum is the name of the pencil pushers of the Imperium of Man. Founded by Malcador the Sigillite back in the day the Emprah was strutting about, the Administratum is the bureaucratic division of the Adeptus Terra. They assess the tithes every world must pay to the Imperium, take census data, and maintain records. Unfortunately, ten thousand years of bureaucracy has created red tape that stretches for light years and inertia that can (and has) cost wars. Imagine the Department of Motor Vehicles with all the grimdark of Warhammer 40,000 and half of the productivity; then multiplied by the number of planets in the Imperium. The Adepts of a department will keep going about their duties long after their department has become completely useless. That's the Administratum. It's run by the aptly named Master of the Administratum, who is the most powerful of the High Lords of Terra.

Divisions

• Departmento Munitorum: The section of the Administratum charged with running the Imperial Guard, but the Guard is so massive that even they cannot give an exact number on how large it is. The Munitorum decides which planets have met their tithe requirements or not, but on a few occasions rounding errors or misspellings have resulted in worlds being declared traitors or wiped out regiments being sentenced to death posthumously.
• Estate Imperium: Charged with maintaining the records of the Imperium. The Chancellor of the Estate Imperium can occasionally get a spot on the High Lords of Terra.
• Historical Revision Unit: The propaganda branch of the Imperium, charged with making sure Imperial history makes the Emperor and the Imperium look perfect and eliminating all knowledge of traitors. Because the Imperium wasn't grimdark enough, they cut/paste the Ministry of Truth into one of their foundational institutions. However, they must have done it with Linux, because they didn't manage to get enough of in to really affect the universe.
• Officio Medicae: The department charged with medical affairs of the Imperium. Unless you're a noble, Guardsman, or Inquisitor, you won't be getting access to any medical assistance without visiting a shitty backalley doc.