The Collegia Titanica is the division of the Adeptus Mechanius that commands the Titans - colossal robotic combat walkers which are the most potent ground weapons capable of being deployed by the Imperium. The Collegia was one of the groups of the Adeptus Mechanicus that went over to the Warmaster Horus almost en masse alongside the Legio Cybernetica during the Horus Heresy. The name "Adeptus Titanica" is also used for the corps of Titans fielded by the Mechanicus.
The Collegia is divided into divisions, such as the Divisio Militaris, Divisio Mandati, Divisio Telepathica and the Divisio Investigatus. The most important of these divisions is the Divisio Militaris since it actually includes the Battle Titans, and is itself further divided into the Titan Legions: groupings of Titans such as the Traitor Death's Head Legion, the Fire Wasps Legion or War Griffons Legion. Each Titan Legion is commanded by an officer known as the Grandmaster who may also be the Princeps of the most powerful Titan in the Legion.
Each Titan is manned by a crew consisting of a single Princeps in command of the Titan, assisted by varying numbers of Moderati, Sensori, Steersmen, Tech-priests, Enginseers, and Servitors. The number of each is dependent on the type of Titan.
Princeps: Princeps training begins at an early age and ends with eight years at the Collegiate Titanica on Mars. The cadets receive their uniforms and cybernetic MIU implants in the final year, before field-training starts with an attachment to a currently-operating Titan for observational tours. A Princeps commands his Titan through a MIU (Mind Impulse Unit, see above) cybernetic link to the Titan's artificial intelligence or "machine spirit", controlling both his crew's actions and the Titan's movements through his thoughts. The Titan becomes an extension of him; when damaged, he feels pain, when victorious, he feels elation. This link grows so strong that his mental health will slowly deteriorate when unlinked from the machine. He can assume direct control of any system, though it is usually aiming and fire control that is taken.
In a Warlord or Imperator-class Titan the Princeps is often placed in an amniotic casket to provide full sensory immersion in order to control the Titan without difficulty. Regardless, Titans must have both a Princeps and a command crew to function. The Princeps' relation to the Titan is similar to that of a Space Marine within a Dreadnought cybernetic shell. However, Titans are capable of eventually developing their own sentient minds from centuries of experience. The cybernetic link in some cases gets so strong that the Princeps becomes neurologically integrated with the Titan, and cannot be removed until he dies of natural causes. A Princeps who commands a Legion of Titans is a "Princeps Maximus", though Titans are rarely fielded as an entire Legion in games of Warhammer 40,000 Epic.
Moderati: The Moderati are the officers who comprise the command crew of a Titan that take the Princeps' orders and act on them, monitoring the Titan's auspices and scanners, firing the weapons and making sure all systems are functioning normally. Although they also have MIU links to the Titan like the Princeps, they cannot assume command nor can they interface directly with the Princeps' own dominant neural link, instead using their links solely to provide information about the Titan's state in battle.
Sensori: The third-ranking member of the bridge command crew, the Sensori is the Titan's Tactical Officer and controls and monitors all the sensory inputs of a Titan, from the aural sensors to the simple visual inputs. It is also his job to spot and acquire targets for the Moderati (or Princeps) to engage. They also deal with all Titan communications, decrypting or relaying vox messages to the Princeps' display screen on his command throne as necessary.
Steersman: The fourth member of the bridge crew. The Steersman controls the Titan's stability and is essentially the driver of the Titan.