Written during the reign of Hadrian, by Hadrian personal secretary, Suetonius, "The Twelve Caesars" was the most popular and longest of Suetonius's surviving works. Beginning with the life of Julius Caesar and continuing with the first eleven emperors of Rome who followed him, The Twelve Caesars is one of the most important historical bibliographical works of the Roman Empire and discusses the critical period in Roman history known as the "Principate," from the end of the Republic to the reign of Domitian.