The Imperium of Man is the galactic empire under which the majority of humanity is united. The founder and ruler of the Imperium is the god-like Emperor, the most powerful human psychic to date. Founding the Imperium in the 30th millennium, he continues, at least nominally, to lead it – although he has been a physically incapacitated husk attached to a complex life-support system for 10,000 years, leaving the High Lords of Terra to rule.
The Imperium is the largest and most powerful political entity in the galaxy, consisting of at least a million worlds, which are dispersed across most of the Milky Way galaxy. Consequently, an Imperial planet might be separated from its closest neighbor by hundreds or thousands of light years. As a stellar empire, the size of the Imperium can not be measured in terms of continuous territory, but only in the number of planetary systems in its control.
Several alien species and other forces (such as the Orks and the powers of Chaos) challenge the supremacy of the Imperium. From within, the Imperium is threatened more insidiously by rebellion, mutation, dangerous psykers, and subversive cults. It is a dystopian nation ever on the verge of disaster.