The Isola Sacra occupies the land between Ostia and Portus at the mouth of the Tiber, and thus lies at the centre of the massive port complex that served Imperial Rome. The area has been the focus of archaeological research since the 16th century, but has never before been the subject of an integrated survey. This volume focuses on the results of a survey completed between 2002 and 2012 as part of the Portus Project. It complements our previous survey of Portus (2005) and the forthcoming publication of the German Archaeological Institute’s survey of Ostia. It included a fluxgate gradiometer survey of all the available open ground on the island (c 98 ha), as well as selective Ground-Penetrating Radar survey and an analysis of satellite and aerial photographic images. A presentation of these results is set against an analysis of the geomorphology of the delta, and integrated with information from past excavations in the area. It is complemented by a programme of geoarchaeological coring and a short account of the ships excavated on the Isola Sacra in 2011. These results are key for understanding the development of Portus-Ostia complex, and hence the economy of the City of Rome itself.