The gensis of this volume, the fourth in the noted CIESM Atlas series of Exotic Species, required years of work and analysis by our foremost experts on Mediterranean algae. One major challenge for the authors was to keep up with a rapidly changing scene in a basin already ranked as a marine biodiversity hotspot and now receiving an exponential influx of newcomer species. As a consequence, the Mediterranean Sea has become the marine region harbouring the largest number of introduced macrophytes in the world. This volume presents, in a form accessible to the largest readership, the most accurate picture possible, based on the latest taxonomic knowledge, of the geographic distribution of marine exotic flora now settled in Mediterranean waters. This involved the careful exclusion of a number of potential candidate species, also detailed here. The reader will find here an illustrated record - complete with distribution maps and key references - of 118 exotic macrophytes that have settled in Mediterranean waters, with the addition of 22 other species now established on the north-eastern Atlantic coast and likely to be the next settlers. Since the influx of exotic species in the Mediterranean Sea is an ongoing process, the taxonomic panorama described in these pages is bound to evolve. Regular updates of future arrivals and geographic extensions of marine macrophytes in our region, documented and validated by the authors, will be found at regular intervals on our Commission website. Chances are that this Atlas will become an indispensable reference for phycologists for years to come. It deserves to be widely read by all those interested in Mediterranean life and, more broadly, by anyone following the fast changing mosaic of marine biodiversity in the world ocean.