The methodology for assessment of Large Marine Ecosystems (LMEs) was developed under the Global Environment Facility (GEF) medium size project (MSP) ‘Development of the Methodology and Arrangements for the GEF Transboundary Waters Assessment Programme (TWAP)’. Large Marine Ecosystems, 64 of which have been defined globally, are natural regions of coastal ocean space encompassing waters from river basins and estuaries to the seaward boundaries of continental shelves and seaward margins of coastal currents and water masses. They are relatively large regions of 200 000 km2 or more, the natural boundaries of which are based on four ecological criteria: bathymetry, hydrography, productivity, and trophically related populations.