The Roman Empire was the post-Republican state of ancient Rome. It is generally understood to mean the period and territory ruled by the Romans following Octavian's assumption of sole rule under the Principate in 27 BC. It included territories in Europe, North Africa, and Western Asia and was ruled by emperors. The fall of the Western Roman Empire in 476 AD conventionally marks the end of classical antiquity and the beginning of the Middle Ages.

The Roman Republic was the era of classical Roman civilization beginning with the overthrow of the Roman Kingdom and ending in 27 BC with the establishment of the Roman Empire. During this period, Rome's control expanded from the city's immediate surroundings to hegemony over the entire Mediterranean world.