The Twelve Caesars, is a set of twelve biographies of Julius Caesar and the first 11 emperors of the Roman Empire written by Gaius Suetonius Tranquillus. The work, written in AD 121 during the reign of the emperor Hadrian, was the most popular work of Suetonius, at that time Hadrian's personal secretary, and is the largest among his surviving writings. The Twelve Caesars is considered very significant in antiquity and remains a primary source on Roman history.