results of the Zea Harbour Project’s investigations of the harbour fortifications at Mounichia in 2005 and the period 2007 to 2012 and to re-examine what can be known about the fortifications of Kantharos, which continues to serve as the Piraeus’ principal harbour to the present day. Chapter 1 (Introduction) considers the objectives, textual sources, archaeological evidence, terminology, and methodology involved, introducing two new terms: “fortified mole” and “fortified breakwater.” Chapter 2 presents the textual evidence relating to the fortifications of the Piraeus, contextualising these structures within the history of the Piraeus from the late 6th century BC until their destruction by Sulla’s forces in 86 BC. Chapter 3 offers an overview of the documentation created by travellers since 1589, topographers since 1686, and scholars since 1881, focusing on their descriptions of the structural remains of the harbours in the Piraeus, but also bringing relevant cartographic, photographic, and artistic documents into consideration. Chapter 4 explores the topography of the fortifications at Kantharos Harbour based on published data. Chapter 5 does likewise for Mounichia, drawing on the findings of the Project’s fieldwork there to provide a detailed analysis of the harbour fortifications’ architecture, supported by feature catalogues of the northern and southern fortified moles. These monumental structures were probably initially constructed in the early 5th century BC. Two later building phases belonging to the 5th century BC (in all probability 429/28 BC or shortly thereafter) and the 4th to 3rd centuries BC have been identified.