A Roman Emperor was the ruler of the Roman Empire, a vast territory that existed from 27 BC to AD 476 in the West and until 1453 in the East. The title of emperor was established by Augustus, the first emperor, who transformed the Roman Republic into an autocratic regime. Emperors held supreme power over the military, government, and religion, often seen as divine figures. Emperors were responsible for significant developments in law, architecture, and culture. Notable emperors include Nero, known for his tyrannical rule, and Constantine the Great, who embraced Christianity and founded Constantinople. The role of the emperor was central to the administration and expansion of the empire.