This Companion is the first of its kind on the Roman historian Cassius Dio. It introduces the reader to the life and work of one of the most fundamental but previously neglected historians in the Roman historical cannon. Together the eighteen chapters focus on Cassius Dio's background as a Graeco-Roman intellectual from Bithynia who worked his way up the political hierarchy in Rome and analyzes his Roman History as the product of a politically engaged historian who carefully ties Rome's constitutional situation together with the city's history.