The seas are changing. Marine conservation seeks to protect valuable habitats but the pristine state of marine ecosystem functioning and biodiversity – that is, the system as it operated before there was any large scale human impact – is conjectural. Conservation management strategies are often based on highly altered ecosystems where the degree of human-induced change is unknown. In SEACHANGE, we propose a structured and systematic approach to the reconstruction of marine ecosystem baselines to quantify the impact of anthropogenic cultural transitions on marine biodiversity and ecosystem functioning. SEACHANGE will address two key questions: 1) What was the nature of long-term changes in prehistoric marine biodiversity and ecosystem functioning over a 3000-year period in NW Europe and the degree of human impact associated with major socioeconomic changes across the Mesolithic-Neolithic boundary? 2) What has been the scale and rate of marine biodiversity loss and changes to ecosystem functioning as a result of fishing intensity and marine habitat loss during the last 2000 years (including the Industrial Transition) in the North Sea and around Iceland, eastern Australia and the west Antarctic Peninsula? To address these questions we will analyse: 1) absolutely-dated annually-resolved bivalve shell series (“sclerochronologies”); 2) marine sediment cores; 3) archaeological midden (waste) materials including shells and bones. We will date these samples precisely and undertake zooarchaeological and palaeoecological, stable isotope geochemical and environmental DNA/DNA metabarcoding analyses. We will compare the data with historical and archival sources, and we will generate numerical ecosystem simulations. We will identify how depleted the current marine environment is compared with that before large scale human impact and what measures are needed, and how long will it take, for marine biodiversity to recover.