Abstract: The Hydrozoa, an inconspicuous taxon of invertebrates, contributes significantly to the bulk of biodiversity but, usually, is noticed only by specialised taxonomists. The taxon Hydrozoa of the phylum Cnidaria comprises about 3800 nominal species, about which knowledge has greatly progressed in recent decades due to the scientific research of some specialists, particularly in the Mediterranean area. Past and present-day information on the global geographic distribution of hydrozoan species was analysed and compared to select those species that have shown fluctuations and geographic shifts in relation to warming of oceanic and marine waters and that can play a powerful and functional ‘sentinel-role’ of global warming. The growth of knowledge of the occurrence and distribution of hydrozoan species is attributable to the labours of numerous zoologists and naturalists, from the beginning of the eighteenth century onwards through many investigations at sea by geographical and oceanographic expeditions. Moreover, the expedition collections were frequently sent to different researchers who tended to specialise either in medusae or in hydroids. The defects of this dual system and large gaps in knowledge of relationships between medusae and hydroids have remained for a long time. Biogeography of the Hydrozoa is a broad field of inquiry combining different approaches, such as the evolutionary (biogeographical) and contemporary (ecological) ones. Nonetheless, numerous efforts have been made to bridge this gap and we are closer to a biogeographical synthesis than ever before. Therefore, initiatives to promote integration among disciplines should be encouraged to achieve a synthetic and comprehensive science of zoogeography.