Modern Estonia is a fairly ethnically heterogeneous country, but this heterogeneity is not a feature of much of the country as the non-Estonian population is concentrated in two of Estonia's counties. Thirteen of Estonia's 15 counties are over 80% ethnic Estonian, the most homogeneous being Hiiumaa, where Estonians account for 98.4% of the population. In the counties of Harju (including the capital city, Tallinn) and Ida-Viru, however, ethnic Estonians make up 60% and 20% of the population, respectively. Russians make up 25.6% of the total population but account for 36% of the population in Harju county and 70% of the population in Ida-Viru county.

What number of Estonia's counties are over 80% ethnic Estonian?