The monograph summarizes the results of many years of research by the author on the problem of the Slavic-Russian development of the northern regions of Eastern Europe and the interaction of the newcomer population and the indigenous Finno-Ugric peoples. It examines in detail the large regions of Ancient Rus', the population of which (chronicles Vod, Izhora, Ves, Chud Zavolochskaya, Merya, Muroma, Meshchera) came into close contact with the Slavs and underwent complete or partial assimilation.