We know little about how ethnic geography, i.e., the distribution of ethnic groups across space, shapes comparative economic, political and social development. To make progress and to harness the growing availability of spatially explicit data, we need indices summarizing key aspects of ethnic geography. We develop and axiomatize a novel index of ethnic segregation that takes both ethnic and spatial distances between individuals into account. We can decompose this index into indices of generalized ethnic fractionalization, spatial dispersion, and the alignment of spatial and ethnic distances. For our application, we compute different country-level versions of the segregation index and its components based on either ethnographic maps or geo-referenced survey data. Reassuringly, the different versions of the segregation index are highly correlated. We explore the relation of our indices to (i) existing measures of ethnic segregation and diversity; (ii) climatic and geographical factors; and (iii) the quality of government, economic development, and trust.