In the fundamental monograph "Old Russian Nationality," which belongs to the pen of the late academician V.V. Sedov, the period of the X-XIII centuries is considered, when the Slavic population of Eastern Europe constituted a single ethno-linguistic community - the Old Russian nationality. On the basis of modern historical and archaeological data, the processes of formation and development of this ethnic group are investigated. The complex picture of the Slavs' development of the East European Plain at the beginning of the Middle Ages and the integration phenomena that led to the unification of different tribal formations into a single community are analyzed.