This book is the first comprehensive and up-to-date fauna covering the Mediterranean Hydrozoa: hydroids, hydromedusae and siphonophores comprised. A general introduction to the Hydrozoa is given followed by keys for identification, diagnoses and illustrations for all the presently known Mediterranean hydrozoan species. Definition of the supra-specific taxonomical categories, short zoogeographical indications and references are provide. This volume represents a useful and important tool for all marine researchers interested in biodiversity, mainly in the Mediterranean area