The Roman Empire in the east transformed into the Byzantine Empire over time, so it's pretty hard to neatly separate the histories of the two empires, but most scholars agree that Emperor Constantine's reign was the start of the Byzantine Empire.
The Byzantine Empire was a vast and powerful civilization with origins that can be traced to 330 A.D., when the Roman emperor Constantine I dedicated a “New Rome” on the site of the ancient Greek colony of Byzantium. Though the western half of the Roman Empire crumbled and fell in 476 A.D., the eastern half survived for 1,000 more years, spawning a rich tradition of art, literature and learning and serving as a military buffer between Europe and Asia. The Byzantine Empire finally fell in 1453, after an Ottoman army stormed Constantinople during the reign of Constantine XI.
