The book is devoted to the study of the nodal problems of the ethnic history of the Mongolian and Buryat peoples. Based on the expeditionary materials collected by the author in northeast China, as well as other sources, the question of finding the Ergune-kun area, with which the initial stages of ethnogenesis and the early history of the Mongols are associated, is investigated. Ethnic and closely related socio-political processes in Mongolia on the eve and during the existence of the empire are analyzed.