Coastlines can be considered as blurred boundaries, as a variable space between land and water, linked to a temporal spatial relationship and influenced by both human and environmental factors. Between Late Antiquity and the Early Middle Ages, the coastal areas of the Campania region underwent considerable transformations. By emphasising the aspect of littoral vulnerability, this chapter attempts to broadly trace these natural and anthropic changes in a long-term perspective extending from the Roman period to the ninth century. The aim is to reflect on the consequences that these changes generated, first of all in the face of the growing Muslim presence. The starting point of the analysis is a request for naval aid addressed by John VIII to Amalfi to protect the papal coast.