Civil wars tend to cluster in particular areas of the world. We provide empirical evidence that cross-border conflict spillovers are an important factor in explaining this pattern. Moreover, we show that ethnicity plays a key role in conditioning the spread of civil wars. Only ethnic wars tend to spill over, and ethnic wars are more likely to spill over along ethnic lines. The latter result is robust to the inclusion of a host of (other) cross-border characteristics, such as geographical factors and trade intensity. We estimate that a neighboring ethnic civil war increases the risk of an outbreak of ethnic civil war on the home territory by 4–6% points.