The Adeptus Terra, also known as the Priesthood of Earth, is the central bureaucratic governmental organisation of the Imperium of Man and not really an agency itself. It is made up of many autonomous departments that receive the orders of the High Lords of Terra, passing them down to the different branches that will enact their commands. There are only a few organisations in the whole of the Imperium that do not fall under this central command, most notable of which are the Inquisition, the Adeptus Mechanicus and the Adeptus Ministorum, also called the Ecclesiarchy. The Adeptus Astartes are officially a part of the Adeptus Terra, but remain a high level of autonomy.
The priesthood which serves the Lord of Humanity is often referred to as "the right hand of the Emperor." It falls to the Adeptus Terra to interpret the Emperor's will and minister to the Imperium. Many hundreds of thousands of souls labour across the galaxy to serve this vast organisation. There are numerous ancient institutions that make up the priesthood, each with various names on various planets. These varying Adepta, as they are traditionally known, each have a specific function to carry out in the Emperor's name. Only the Ecclesiarchy and the Imperial Inquisition are not formally a part of the Adeptus Terra.
The Adeptus Terra is an umbrella organisation, and not really an agency in itself, as each of its divisions functions largely autonomously and separately from the others. The most powerful individual in the Adeptus Terra (and if not in theory, then in practice, the entire Imperium) is the Master of the Administratum, who controls the central, secular bureaucracy of the Imperium.
The following Adepta of the Imperial government are all formally part of the Adeptus Terra:
- Adeptus Administratum
- Adeptus Astra Telepathica
- Adeptus Astronomica
- Adeptus Arbites
- Adeptus Astartes
- Adeptus Custodes
- Logos Historica Verita
- Codex: Imperialis (2nd Edition)
- Codex: Militarum Tempestus (6th Edition), pp. 5-13
- Dark Imperium (Novel) by Guy Haley, Ch. 9 "Imperator Gloriana," pp. 94-97
- Warhammer 40,000: Rulebook (8th Edition), pp. 24-25
- Warhammer 40,000: Rulebook (Digital Edition) (7th Edition), pp. 275, 278
- Warhammer 40,000 Rulebook (6th Edition), pg. 139