The Principate was the form of imperial government of the Roman Empire from the beginning of the reign of Augustus in 27 BC to the end of the Crisis of the Third Century in AD 284, after which it evolved into the Dominate. The principate was characterised by the reign of a single emperor (princeps) and an effort on the part of the early emperors, at least, to preserve the illusion of the formal continuance, in some aspects, of the Roman Republic.

The Dominate, also known as the late Roman Empire, is the despotic form of imperial government of the late Roman Empire. It followed the earlier period known as the Principate. Until the empire was reunited in 313, this phase is more often called the Tetrarchy.