Flavius Valerius Aurelius Constantinus, also known as Constantine the Great, was a Roman emperor who ruled from 306 to 337 AD. He is best known for being the first Roman emperor to convert to Christianity and for issuing the Edict of Milan in 313, which granted religious tolerance to Christians in the Roman Empire. Constantine also played a significant role in the establishment of the city of Constantinople (modern-day Istanbul) as the new capital of the Roman Empire. His reign marked a major turning point in the history of the Roman Empire and the spread of Christianity.