This paper examines the sources and methods of Appian and Cassius Dio and the likelihood that they used the Histories of Seneca the Elder as a source. It also considers the character and starting point of Seneca’s Histories. Both Appian and Dio probably wrote up their histories from drafts which they compiled from their reading, but in other respects their aims and methods were very different. Cassius Dio’s work was composed in the traditional mode of an annalistic Roman history from the foundation to his own time, while at the same time aspiring to high Greek literary standards. Dio read widely and seems to have drawn on multiple sources for most of his account. For the period which it covered the Elder Seneca’s Histories may have been one of his sources. Appian’s Roman History adopted a radically innovative structure, opening with a twelve-book account of the external wars of the Roman Republic, organized by region rather than linear chronology, and then passing to the violent internal discords initiated by the tribunate of Tiberius Gracchus and culminating in the establishment of monarchy, followed by a concluding overview of the wars of the imperial period. Unlike Dio, Appian seems to have followed a single main source for much of his work, including Dionysius for events down to 265, Polybius for events from 200 to 146 BC, and perhaps Posidonius for the immediately following period. For his more ample narrative after Caesar’s death he seems to have used a wider range of sources, which may possibly have included the Elder Seneca. Appian’s structure had much in common with that of Florus’ much shorter history. The earlier of these writers must have influenced the later, but their priority cannot be determined. The view that the Elder Seneca’s Histories served as the model for the structural features shared by Appian and Florus should be rejected. The starting point of Seneca’s Histories was probably the war between Caesar and Pompey. His work had the traditional character of a Roman history limited to the recent past, was probably organized by the consular year, and so had nothing in common with the innovative structures of Appian and Florus.