The author argues that Dio's account of the Principate was based on extensive reading of literary sources, whose precise identification is almost impossible, for he selected and reworked with great independence the copious material he gathered. Moreover, he drew information from systematic reading of documentary sources, above all the archives of the senate, but also the acta urbis and inscriptions. He probably employed only a fraction of the material he gathered, in accordance with his methodological principle of omitting ‘meaningless’ details and with his overriding interests: honour and festivities for emperors, as well as institutional, procedural and ceremonial aspects of the Roman state, which he collected with the intention of comparing them with his own time.