When do ethnic civil wars last especially long?This dissertation examines when, why, and how ethnicity affects the length of civil wars.Two-thirds of civil wars pit ethnic combatants against each other; the duration of these conflicts varies considerably.Existing work on ethnicity in civil wars is effectively stalemated on the questions of how or even whether ethnicity influences the lengths of these wars.Yet the answer has vital normative, policy, and research implications.In this dissertation, I argue that ethnicity will prolong civil wars under two conditions.First, when information derived from ethnic interactions exacerbates combatants’ fears of the future, conflicts will last longer.Second, when support from ethnic kin in other states alters the balance of capabilities or introduces uncertainty into wars, conflicts will be protracted.Using duration analysis of a new dataset of all ethnic civil wars from 1945 to 2004, I show that both of these dynamics prolong ethnic civil wars.Case studies from two post-Soviet republics demonstrate that ethnic interactions are especially likely to prolong conflicts when they exacerbate commitment or signaling problems.Two civil wars in Indonesia show that ethnic kin are especially influential when they influence the balance of capabilities.Case studies of civil wars in Central America and Sri Lanka indicate that these findings also may have implications for both variation within non-ethnic civil wars and between ethnic and non-ethnic civil wars.