When was the Roman Empire?
The Roman Empire lasted from 27 BCE, when Augustus became the first emperor, to 476 CE, when the last Western emperor was deposed. The Eastern Roman (Byzantine) Empire continued until 1453 CE. At its height under Trajan (117 CE), the empire encompassed the entire Mediterranean world, from Britain to Mesopotamia.
The Roman Empire's dates depend on which definition you use. The traditional dating places its beginning at 27 BCE, when Octavian received the title Augustus from the Senate, establishing the Principate. Its end is conventionally dated to 476 CE, when the Germanic chieftain Odoacer deposed Romulus Augustulus, the last Western Roman emperor.
Within this 500-year span, the empire went through distinct phases. The Principate (27 BCE – 284 CE) saw the Pax Romana, the empire's greatest territorial extent under Trajan (117 CE), and eventually the Crisis of the Third Century. The Dominate (284–476 CE), beginning with Diocletian's administrative reforms, saw more openly autocratic government, the adoption of Christianity, the permanent division of the empire (395 CE), and the gradual fall of the Western half.
The Eastern Roman Empire — known to modern historians as the Byzantine Empire — survived the fall of the West by nearly a thousand years, enduring until the Ottoman conquest of Constantinople in 1453 CE. The Byzantines considered themselves Romans and maintained Roman legal traditions, administrative structures, and cultural heritage throughout their existence.
If we include the Roman Republic (509–27 BCE) and the Byzantine continuation, the total span of Roman civilization stretches from 509 BCE to 1453 CE — nearly two millennia. This extraordinary longevity is one reason why Rome's legacy is so pervasive in Western and Mediterranean civilization.