The Imperium considers humanity the rightful rulers of the whole galaxy. Part of the Imperial Creed is that all aliens judged to pose a threat to mankind's survival must be contained or destroyed. Aliens, along with mutants, heretics and deviants, are all considered enemies of mankind, for they threaten the very survival of the human race.
Many new alien races and civilisations are discovered by Imperial Explorators every year. For the most part, these aliens are ignored, contained, or if they present a threat to mankind or the Imperium, eradicated. Throughout its history the Imperium has fought many long wars with dangerous alien species. In most cases the Imperium has eventually won, but in some, the threat has only managed to be contained.