The Adeptus Astartes (commonly known as Space Marines, and colloquially as Angels of Death) are one of the most elite and feared fighting forces in the Imperium of Man. The primary unit of organisation is the Chapter, a self-contained army fully equipped with its own transport, non-combatant support staff, etc. There are around a thousand Chapters, each comprising a thousand Space Marines. There are far too few Space Marines to form the Imperium's main military forces; instead they operate as highly mobile strike forces. They are entrusted with the most dangerous missions, such as lightning raids behind enemy lines, infiltration, and tunnel fighting. Through selection, training, and conditioning, Marines are physically, mentally, and spiritually superior to any other Imperial soldier.