The present-day national structure of Slovakia is, among others, the result of a long-term population and residential development, to a high degree conditioned by migrations, but also by political interventions from above that also influences the formation of linguistic frontiers and regions. The study aims to present a general overview of the ways how ethnicity (ethnic identity) was perceived from the point of view of statistics (official state censuses) to characterize the basic sources for the study of ethnicities in Slovakia and thus to sketch the ethnic composition of Slovakia at the end of the 19th and the beginning of the 20th century according to the atributes valid and observed in the studied period.