Marine sediments dominate the global seabed, creating the largest ecosystem on earth. Seafloor biodiversity is a key mediator of ecosystem functioning, yet critical processes are often excluded from global biogeochemical budgets or simplified to black boxes in ecosystem models. This accessible textbook provides an ideal point of entry into the field, providing basic information on the nature of soft-sediment ecosystems, examples of how and why we research them, the new questions these studies inspire, and the applications that ultimately benefit society. While focussing on coastal habitats (