Medusozoa, commonly known as jellyfish, are a subtype of gelatinous zooplankton. The iconic medusa-phase represents the sexual phase within their complex life cycle. Many different subphyla are found around the world, from shallow costal waters to the deep sea. Under favourable conditions, which are not yet fully understood, they can reproduce rapidly. The resulting event is known as a ‘jellyfish bloom’. Those large swarms may have a negative impact on aquaculture industry, tourism and even clog cooling systems in coastal power plants. To investigate this further, the European research programme GoJelly, coordinated by Geomar, Kiel, has been initiated. Its aim is to find ways to predict the occurrence of jellyfish blooms, as well as to find use-cases for the resulting vast amounts of biomass.