In the Warhammer 40,000 universe, the Adeptus Mechanicus is a techno-cult that maintains the ancient technology of the Imperium of Man. Over many thousands of years, Terran science and knowledge have given way to religion and mysticism. Engineers have been replaced by priests who use rituals and prayer to appease or heal a machine's spirit. Because most of the underlying knowledge has been lost, Adepts cannot re-create technology (such as a Dreadnought) and must carefully preserve what still exists.
The Adeptus Mechanicus is based on Mars, and many Space Marine chapters will send individuals there to study the ancient ways. Anyone associated with the Mechanicus Cult is regarded with a mixture of suspicion and respect by outsiders.