This chapter is a reinterpretation of the Roman frontier in Late Antiquity. In the last decades, historians have described frontier rivers as primarily facilitating communication and cultural contact and less as borders of exclusion; contrariwise, archaeologists still concentrate their efforts on the military dimension. In many ways this dichotomy was engendered by insufficient conversation between historians and archaeologists and has delayed the development of a conceptual model which could help bridge such disciplinary divides. The chapter is an attempt to offer such a model, drawn from the anthropological study of frontiers. By doing so it is necessary to intrude upon several of the major questions of Byzantine history. What was the Byzantine worldview on frontiers? Is there any change from the early Roman centuries? Was there a Grand Strategy in Late Antiquity? A careful comparative study reveals enduring literary topoi but also an ongoing concern to use reinforced natural obstacles as political frontiers able to act as convenient barriers against “barbarians.” This chapter re-emphasizes the strategic role of the Danube in Late Antiquity and its political function of separation.