Article dans une revue: We provide a new compilation of data on ethnic, linguistic, and religious composition at the subnational level for a large number of countries. Using these data, we measure segregation of groups within the country. To overcome the endogeneity problem that arises because of mobility and endogenous internal borders, we construct an instrument for segregation. We find that more ethnically and linguistically segregated countries, i.e., those where groups live more spatially separately, have a lower quality of government; there is no relationship between religious segregation and governance. Trust is an important channel of influence; it is lower in more segregated countries.