This chapter explores the nature of civil conflict in the Greco-Roman world from the Greek polis or city-state through to the Roman republic, and then Empire. The concept of 'civil war' owes much to the Roman conception of bellum civile – or citizens war, but similar events can be found earlier in the Greek world too. In relation to the Greek polis, civil conflict – or stasis – arose out of conflict around constitutional forms, the nature of political participation, and socio-economic concerns. Such conflict could rise to the level of war, but not necessarily so. In the Roman republic, there was generally large degree of civil stability until the last century of the republic, when the pressure of a Mediterranean-wide empire caused socio-economic dislocation, and elite competition frayed political consensus. In Hellenistic monarchies, and the early Roman empire, civil conflict often arose as a means of settling uncertain succession. In the third century CE, heightened external pressures strained existing weaknesses in the Roman system, leading to civil war over who should be emperor. Although 'ethnic cleansing' has often been seen in modern civil conflict, most violence in the Greco-Roman world in civil conflict was politically, rather than ethnically based. That said, ethnically-based violence in the Greco-Roman world was intimately connected with war more generally.