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 ten thousand years ago, he continues, at least nominally, to lead it.
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 neighbour 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 aliens and forces (the forces of Chaos, Tyranids, Eldar, Dark Eldar, Orks, Tau, Necrons) challenge the supremacy of the Imperium. From within, the Imperium is threatened more insidiously by rebellion, mutation, dangerous psykers, and subversive cults.
Without the protection of the Imperium, mankind would fall prey to the countless perils that threaten it.