Civil war is an armed conflict between the state and another organized domestic party over a contested political incompatibility, which results in a number of casualties exceeding a certain threshold for both parties. Attempts to operationalize these criteria have produced many data sets, which conceptualize civil war as distinct from one-sided violence, organized crime, and communal fighting. Civil wars are devastating for states experiencing them, their neighbors, and the entire global community. Combatant and civilian deaths, rape, massive refugee flight, negative impacts on economy and infrastructure, spread of infectious diseases, global spread of illegal narcotics, and the promotion of terrorism are all consequences of civil wars. Theories explaining why civil wars occur focus on objectives of the rebels, ability of rebels to successfully challenge the government, influence of external actors on interactions between the government and the opposition, external financing of potential rebel groups, and impact of a state’s neighbors on the likelihood of civil conflict or how neighboring conflicts and refugee communities serve as breeding grounds for cross-border rebel movements. Conflicts persist until neither side believes that it can achieve unilateral victory and continued fighting is costly. Governments are more likely to win early when they have large armies, but time to government victory increases when they are faced with secessionist rebels and when external parties are involved. Meanwhile, external mediation diminishes informational and credible commitment problems during bargaining and reduces conflict duration. Promising directions for future research on civil war include geographic disaggregation, survey research, and computational/agent-based modeling.