In an effort to give to Mare Thracium the deserved prominence in its entirety, both to its important settlements as well as to its importance as a transport area, a workshop was held in Mainz in 2016 as part of the Interdisciplinary Archaeological Project »The Thracian harbour city Ainos in Roman and Byzantine times – the development of a traffic hub in a changing environment« which was a participant of the Priority Research Program of the German Research Foundation (DFG SPP 1630) »Harbours from the Roman Period to the Middle Ages«. The contributions in this volume are addressing: natural conditions for seafaring, historical developments, physical remains of coastal settlements and their harbours as well as underwater discoveries.