The High Lords of Terra are but twelve men and women, and so they must confine their attentions to the very highest levels of strategy, policy, and war. An individual planetary ruler might occasionally draw their interest, but even then only briefly. The countless tasks that must be carried out simply to ensure the Imperium continues then fall to any one of numberless institutions into which almost all of the servants of the Imperium are divided. The Adeptus Terra is in effect the government of the Imperium, the overall structure that controls and oversees at the very highest level everything from the conduct of decades-long wars of attrition to the exploration of newly discovered worlds.
The Adeptus Terra is sometimes referred to as the Priesthood of Terra, or simply the Priesthood, for it has a hand in almost every aspect of life, even it is not a specific department in itself. Rather, the Adeptus Terra is divided into a staggering array of divisions. Some are ubiquitous and known to every world in the Imperium, such as the Adeptus Administratum and its endless adepts and scribes. The Astra Militarum, more commonly known as the Imperial Guard, is also ubiquitous as it raises new regiments and defends the Imperium across the galaxy. Others operate in secret, such as the Officio Assassinorum, its agents moving unseen and visiting death wherever the High Lords will it. Some are figures of legend, such as the Space Marines of the Adeptus Astartes, the "greatest of champions", who most men will never be blessed to witness. Still more divisions exist, their purpose so esoteric or anachronistic as to defy reason.