This volume provides a systematic and zoogeographical account of the medusae and siphonophores collected in southwestern Africa during the cruises carried out by the Institut de Ciències del Mar (Barcelona) and the Sea Fisheries Research Institute (Cape Town). Descriptions, illustrations and information on distribution are reported for 116 species. Planktonic cnidarians are one of the main groups of the macrozooplankton communities. Little is know about their abundance and diversity in marine areas. This volume represents an important increase in the knowledge of one of the most productive and interesting marine regions: the Benguela System. This book provides useful information not only for cnidarian scientists but also for zooplankton and marine invertebrate researchers.