The Roman Empire was the post-Republican period of ancient Rome. As a polity it included large territorial holdings around the Mediterranean Sea in Europe, Northern Africa, and Western Asia ruled by emperors. From the accession of Caesar Augustus to the military anarchy of the 3rd century, it was a principate with Italy as metropole of the provinces and the city of Rome as sole capital (27 BC – AD 286). After the military crisis, the empire was ruled by multiple emperors who shared rule over the Western Roman Empire and over the Eastern Roman Empire (also known as the Byzantine Empire). Rome remained the nominal capital of both parts until AD 476, when the imperial insignia were sent to Constantinople, following the capture of Ravenna by the barbarians of Odoacer and the subsequent deposition of Romulus Augustulus. The adoption of Christianity as the state church of the Roman Empire in AD 380 and the fall of the Western Roman Empire to Germanic kings conventionally marks the end of Classical antiquity and the beginning of the Middle Ages. Because of those events, along with the gradual hellenization of the Eastern Roman Empire, historians distinguish the medieval Roman Empire that remained in the Eastern provinces as the Byzantine Empire.

The predecessor state of the Roman Empire, the Roman Republic (which had replaced Rome's monarchy in the 6th century BC) became severely destabilized in a series of civil wars and political conflicts. In the mid-1st century BC, Julius Caesar was appointed as perpetual dictator and then assassinated in 44 BC. Civil wars and proscriptions continued, eventually culminating in the victory of Octavian, Caesar's adopted son, over Mark Antony and Cleopatra at the Battle of Actium in 31 BC. The following year Octavian conquered Ptolemaic Egypt, ending the Hellenistic period that had begun with the conquests of Alexander the Great of Macedon in the 4th century BC. Octavian's power then became unassailable, and in 27 BC the Roman Senate formally granted him overarching power and the new title Augustus, effectively making him the first Roman emperor.

The first two centuries of the Empire saw a period of unprecedented stability and prosperity known as the Pax Romana ("Roman Peace"). Rome reached its greatest territorial expanse during the reign of Trajan (AD 98–117). A period of increasing trouble and decline began with the reign of Commodus (177–192). In the 3rd century the Empire underwent a crisis that threatened its existence, as the Gallic Empire and Palmyrene Empire broke away from the Roman state, and a series of short-lived emperors, often from the legions, led the empire. The empire was reunified under Aurelian (r. 270–275). In an effort to stabilize it, Diocletian set up two different imperial courts in the Greek East and Latin West in 286. Christians rose to positions of power in the 4th century following the Edict of Milan of 313. Shortly after, the Migration Period, involving large invasions by Germanic peoples and by the Huns of Attila, led to the decline of the Western Roman Empire. With the fall of Ravenna to the Germanic Herulians and the deposition of Romulus Augustus in AD 476 by Odoacer, the Western Roman Empire finally collapsed; the Eastern Roman Emperor Zeno formally abolished it in AD 480. On the other hand, the Eastern Roman Empire survived for another millennium, until Constantinople fell to the Ottoman Turks of Sultan Mehmed II in 1453.