Marine sediments dominate the seafloor, creating one of the largest ecosystems on earth. Marine sediments contain some of the steepest known natural chemical gradients and are extraordinarily productive and reactive, particularly in shallow water. The plants and animals that live on and in marine sediments create highly heterogeneous conditions that strongly influence ecosystem functions and how marine ecosystems drive and respond to change. Seafloor biodiversity is a key mediator of ecosystem functioning, but its role is often excluded from global budgets or simplified to black boxes in ecosystem models. Despite this, marine sediments are fascinating places to study population, community and ecosystem ecology. This book provides an overview of soft-sediment ecosystems and how and why we should study them. It addresses the interactions between marine organisms and their physical and chemical environment, why we need to carefully design research and provides basic steps needed to both formulate good ecological questions and translate them into empirical studies of real-world ecosystems. It provides a context for different points of entry into soft-sediment ecology by offering a high-level approach. It is designed to help you think about the connections between different system components and drivers of change and identify how you can make a contribution to developing knowledge on the biodiversity and functioning of soft sediments and understanding ecosystem change, human impacts and the need for restoration.