Jellyfish, a group that includes scyphomedusae, hydromedusae, siphonophores and ctenophores, are important zooplankton predators throughout the world's estuaries and oceans. These beautiful creatures have come to public attention as featured exhibits in aquaria and in news headlines as invaders and as providers of genes used in biomedical research. Nevertheless, jellyfish are generally considered to be nuisances because they interfere with human activities by stinging swimmers, clogging power plant intakes and nets of fishermen and fish farms, and competing with fish and eating fish eggs and larvae. There is concern that environmental changes such as global warming, eutrophication, and over-fishing may result in increased jellyfish populations. The literature reviews and research papers in this volume explore the interactions between jellyfish and humans.