A Space Marine Chapter is the fundamental organisation of the Adeptus Astartes, a self-contained brotherhood of roughly a thousand superhuman warriors. Since the reforms that followed the great betrayal, the Space Marines have been divided into these smaller formations, each independent and self-sufficient, so that no single commander might ever again wield the catastrophic power of the old Legions.
A Chapter is far more than a fighting force. It maintains its own fleet, its own fortress-monastery, its own armouries and traditions, and above all its own supply of sacred gene-seed, the genetic material from which new Astartes are made. Structured according to hallowed doctrine into companies and squads, it is led by a Chapter Master and served by specialist officers, chaplains, and warrior-monks who preserve its rites and the memory of its heroes.
Each Chapter is a world unto itself, with its own heraldry, war-cries, and jealously guarded character forged over millennia. Some hew closely to the sacred template of their organisation, while others preserve older traditions or strange practices born of their gene-line and homeworld. Roughly a thousand Chapters are thought to exist across the galaxy, each a brotherhood of legends bound by oath to defend humanity. To learn how the old formations became these smaller orders, see the Space Marine Legion.