Humans - Imperium of Man: Astra Militarum - The backbone of Imperial military might in the 41st Millennium, the Astra Militarum, or "Imperial Guard," are not a single, cohesive fighting force but are rather a vast military comprised of a wide spectrum of different equipment, tactics and doctrines depending on the particular star system from where an Astra Militarum regiment originates. Because they are formed in this way, recruited and equipped by the resources available in their particular home star system and home sector, the Imperial Guard are the largest single fighting force in the galaxy and they number in the billions of soldiers, all willing to die in the name of the Emperor of Mankind.Collegia Titanica - The Collegia Titanica is the division of the Adeptus Mechanicus that includes the Legions composed of Titans, the colossal Imperial robotic war machines that are the most powerful engines of war in the Imperium of Man. The Collegia is also more rarely known as the Adeptus Titanicus (a contraction of "Adeptus Mechanicus Collegia Titanica").Adeptus Astartes - The Space Marines are genetically-modified transhuman warriors designed specifically to lead the Human reconquest of the galaxy during the Great Crusade that created the Imperium of Man over ten thousand Terran years ago during the late 30th and early 31st Millenniums. Since the end of the Horus Heresy, the Space Marines have adopted more of a defensive role, fighting in void assaults, ship-to-ship actions and as specialists in planetary assault, as well as aiding the regiments of the Astra Militarum by fighting alongside them when they need help in the most difficult situations. The Space Marines now fight as individual 1,000-warrior-strong Chapters which possess the autonomy to choose which conflicts they will participate in and where they can best aid the defence of the Emperor's realm.Sisters of Battle (Orders Militant of the Adepta Sororitas) - The Sisters of Battle are the least common Human army on the battlefield because they were created specifically to fight Imperial Heretics and rebels. Just as only Human males can be Space Marines, only Human females may serve among the Sisters of Battle. They serve as the military forces of the Adeptus Ministorum, better known as the Ecclesiarchy, the ecclesiastical bureaucracy which administers the star-spanning Imperial Cult of the God-Emperor. The Sisters also serve as the Chamber Militant of the Imperial Inquisition's Ordo Hereticus, tasked with combating the enemies of the Emperor who exist within the Imperium.Navis Imperialis - The Imperial Navy holds nearly all of the Imperium's fighting starships; local governments, Warmasters and others are forbidden to maintain their own fleets of warships. Their spacecraft include some of the most potent engines of destruction in the whole galaxy, including mighty battleships thousands of years old. The Navy's starships range from small escorts with a crew of a few dozen to the immense Emperor-class Battleships which might have 20,000 souls or more on board. The Navy also includes fighter and bomber crews and aircraft of the division known as the Aeronautica Imperialis that support the forces of the Astra Militarum on the ground. The Navy's officer class is highly traditional and aristocratic in character. The Imperium's noble families frequently boast naval officers among their number and naval dynasties dominate many battlefleets. Elitism is a virtue on most ships, where the officers' lives are in stark contrast to those of the ratings and engine crews. With thousands of crew living and dying on ships that can spend solar decades without seeing port, a ship of the Imperial Navy becomes a city in space. Mutinies are not unknown and the naval security battalions of armsmen are a familiar sight on the decks of all naval ships, their black visors and shotguns constantly reminding the crewmen that obedience is their duty to the Emperor. The Navy relies on many other organisations to function. Perhaps most importantly, its ancient and complex warships could not function without a complement of Tech-priests who know how to appease the starships' Machine Spirits and maintain technology that is too old and mysterious to be replicated. The Navy is also reliant on the Tech-priests of Mars for refits, upgrades, repairs and the construction of new vessels. It is usual for a warship to have astropaths on board, for proper communication is essential if the Imperium is to be defended. Many captains are also glad to have Adeptus Ministorum clergy among their crew, ministering to the spiritual needs of their fellow crewmen and steeling their spirits against the dangers of the void with sermons. Commissars of the Officio Prefectus are appointed to serve aboard the larger ships, watching over the moral fibre of the crew and providing a watch against mutiny and impiety. Then, of course, there is the Navigator of each vessel, whose family can occupy entire spires jutting out into space atop an Imperial warship.Adeptus Mechanicus - The Adeptus Mechanicus is the semi-autonomous religious organisation that provides the Imperium with its scientists, engineers and technicians. The Tech-priests of the Cult Mechanicus are the primary keepers of what is viewed as sacred and divinely-inspired wisdom within the Imperium, a privileged caste who jealously guard the knowledge required to maintain and construct much of the Imperium's advanced technology. The Adeptus Mechanicus deploys several types of autonomous military forces to protect their Forge Worlds, to pursue their own interests and to assist Imperial military campaigns when necessary. These include the Cult Mechanicus' own Battle Congregations of armed Tech-priests, the cybernetic troops of the Skitarii Legions, the Battle-automata of the Legio Cybernetica, the Titans of the Collegia Titanica, the Imperial Knights, and squads of combat servitors.