This chapter will take a new approach to the study of migration and mobility to the harbour cities of Ostia and Portus in Late Antiquity. The trade networks of the Mediterranean created a system of seasonal mobility. Thus, temporary populations of the cities, the harbours, and the countryside of Ostia and Portus changed the image of the cities during the year, from the peaceful period of the winter months to the busy period of the summer months with traders, seamen, labour force of the harbours, and visitors arriving from the Roman provinces all over the Mediterranean. The connection and the mutual dependence of the two harbour cities to the metropolitan area of Rome, the Mediterranean area and the provinces of the Roman Empire are discussed and compared to the previous studies of migration in Antiquity and Laurence Tacoma’s definitions of migration to Rome. As Tacoma did not include the analysis of migration to Ostia and Portus in his study, this paper aims to raise the specific character of the harbour cities of Rome to the centre of attention, with an emphasis on the development in Late Antiquity, with the help of archaeological evidence, literary sources and inscriptions, also through a comparison to the latest geological studies of the coastal area of Latium and the Tiber river, and the latest results of ancient DNA studies.