Summary: Civil war is the dominant form of armed conflict in the contemporary international system, and most severe lethal armed conflicts in the post-Cold War era have been civil rather than interstate. Still, it would misleading to see these conflicts as purely domestic, as many contemporary civil wars such as Syria display clear transnational characteristics, including inspirations from events in other countries, links to actors in other countries, as well as often international interventions. Moreover, civil wars often have important implications for other states, including security concerns and economic impacts.This chapter reviews the growth and core findings in the literature focusing on the transnational dimensions of civil war. I focus in particular on how factors outside a particular state can influence the risk of conflict within states as well as some of the central consequences of domestic conflict for other states or relations between states. I conclude that this line of research has helped expand our understanding of both civil conflict and interstate war, and that a comparative focus on varieties conflict and attention to the possible transnational dimensions of civil war deserve a prominent role in future research.