"They shall be my finest warriors, these men who give of themselves to me. Like clay I shall mould them and in the furnace of war I shall forge them. They shall be of iron will and steely sinew. In great armour I shall clad them and with the mightiest weapons shall they be armed. They will be untouched by plague or disease; no sickness shall blight them. They shall have such tactics, strategies and machines that no foe will best them in battle. They are my bulwark against the Terror. They are the Defenders of Humanity. They are my Space Marines...and they shall know no fear." — The Emperor of Mankind

The Space Marines or Adeptus Astartes are foremost amongst the defenders of humanity, the greatest of the Emperor of Mankind's warriors. They are barely human at all, but superhuman; having been made superior in all respects to a normal man by a harsh regime of genetic modification, psycho-conditioning and rigorous training. Space Marines are untouched by plague or any natural disease and can suffer wounds that would kill a lesser being several times over, and live to fight again. Clad in ancient Power Armour and wielding the most potent weapons known to Man, the Space Marines are terrifying foes and their devotion to the Emperor and the Imperium of Man is unyielding. They are the God-Emperor's Angels of Death, and they know no fear.

The Astartes are physically stronger, far more resilient and often mentally far removed from the lot of most normal human beings. In the presence of the Astartes, most people feel a combination of awe and fear, and many cultures on the more primitive worlds simply worship them outright as demigods or angels of the God-Emperor made flesh. They should feel so, for many Space Marines feel little compassion for those they have sometimes termed "mortals" in comparison to themselves, seeing the very people they were created to protect as little more than obstacles to a more efficient eradication of the Imperium's enemies. This is an attitude sometimes taken by whole Chapters. They see normal humans as frail, weak creatures given to the follies of temptation, avarice, greed, lust and cowardice -- all emotions they rarely feel, if ever. Yet there are some Astartes who remember why they were created by the Emperor, who avoid the trap of hubris which the Space Marines are so prone to and which has seduced so many of their number to serve the Ruinous Powers of Chaos. They are the final guardians of Mankind, the saviours of last resort.

20 immortal superhumans blessed with extraordinary intelligence, charisma and sheer physical might who were to be his generals and closest comrades during the Great Crusade to reunite the scattered and long-isolated human colony worlds after the end of the Age of Strife. The Primarchs wielded powers the like of which are not known in the Imperium today, yet they were lost to the Warp in an accident deep within the Emperor's gene-laboratories beneath the fortress that would become the Imperial Palace and were scattered, still in their gestation capsules, to worlds across the galaxy by the will of the Dark Gods of Chaos. The first Space Marines of the nascent Imperium were also the creation of that era, each made using the genetic inheritance of one of the Primarchs, albeit diluted a hundred times, for no merely human body could contain such power. As each of the Primarchs were encountered in turn by Imperial Expeditionary Fleets during the progress of the Great Crusade, they became the natural and obvious leader of the Space Marine Legion created from their genetic material and with whom they had so much in common. In many cases the Primarch's adopted world became the new base of operations for their Legion and was known henceforth as that Legion's homeworld. The Primarchs then recruited their loyal followers from each of these world's peoples into the ranks of their Legion while others were given rights to draw fresh blood from suitable warlike worlds that were liberated as the Great Crusade progressed.

In the wake of the calamity that was the Horus Heresy, the foundations of the present-day Imperium were laid down. The first High Lords of Terra established the structure by which the Adeptus Terra operated, and described the feudal responsibilities and duties of the Planetary Governors. One of the most important accomplishments was the reorganisation of the Imperium's military forces. This was undertaken almost single-handedly by the Primarch of the Ultramarines Legion, Roboute Guilliman, who with his characteristic speed and efficiency codified the structure of the Astra Militarum, the Imperial Navy, and the Space Marines. Of all of his works, the most influential is the Codex Astartes, the great prescriptive tome that lays down the basic organisational and tactical rules for a Space Marine Chapter. The Codex Astartes decreed that Space Marines would be created and trained over a controlled period of time.