This report draws together scientific understanding of deep-water sponge grounds alongside the threats they face and ways in which they can be conserved. Beginning with a summary of research approaches, sponge biology and biodiversity, the report also gives up-to-date case studies of particular deep-water sponge habitats from around the world. These include the spectacular giant glass sponge reefs of British Columbia – a relic of the time of the dinosaurs – and the diverse sponge kingdom of Antarctica. Long overlooked, recent research now shows that deep-water sponge grounds form complex, slow-growing and long-lived habitats in many parts of the global ocean. As well as forming local biodiversity centres, deep-water sponges are also storehouses of novel chemical compounds, some of which show promise in the fight against cancer and other diseases. Despite their inherent and biotechnological value, deep-water sponge grounds have been damaged by bottom fishing. This report considers the international policy context in which deep-water sponge grounds can be conserved and concludes with a series of expert recommendations for conservation managers and international policy makers. The recommendations set out a series of actions so that these vulnerable marine ecosystems can be conserved for future generations.