In this volume the concept of complex systems is used to discuss the interplay between social and environmental factors for the emergence and maintenance of maritime infrastructure and route systems in the ancient and medieval period. The approach is applied to various aspects of maritime history within the research programme ‘Harbours from the Roman Period to the Middle Ages’. Complexity theory and network analysis provide an analytical framework to describe social configurations (cities, maritime communities, polities) and environmental phenomena (hydrosphere, climate) as complex systems, entangled via mechanisms of feedbacks, adaptation or disruption.