Marine biodiversity underpins the health of the oceans and their productive ecosystems which, in turn, support sustainable fisheries and provide enormous possibilities for new biotechnological applications. However, marine biodiversity is changing rapidly as a result of natural and human pressures. Such changes can lead to environmental, economic and ultimately to social problems, but also to new opportunities for people and industry. To explore, conserve, and make better use of marine biodiversity, we need a deeper understanding of its origin, its composition and its role in ecosystem functioning.