This article assesses the role of ethnic demographics and domestic ethnic rebellion in promoting international conflict. Three of the variables introduced examine dyads within which a common ethnic group exists. These variables are coded to distinguish the presence of a trans-border ethnic group that exists as a majority in both states; a majority in one state and a minority in the other; or a minority in both states. The pooled dataset, which covers the years 1951–1991, is analyzed using different types of data to account for both broad and narrow conceptions of ethnicity. The results indicate a strong and significant increase in dyadic conflict when two states share an ethnic group and an ethnic majority exists in at least one of the states. Ethnic rebellion is also found to significantly harm interstate relations when an ethnic diaspora is involved. Associations found between ethnicity and international conflict are most pronounced when international disputes involve fatalities.