Recognizing the value of large marine ecosystems (LMEs) and other transboundary water systems (open ocean, groundwater aquifers, lakes and reservoirs, and river basins), their continued degradation, the fragmented approach to their management, and the need for better prioritization of interventions, the Global Environment Facility (GEF) embarked on the Transboundary Waters Assessment Programme (TWAP) from 2009 to 2015. TWAP objectives were to undertake global assessments of the five transboundary water systems to assist GEF and other international organizations set priorities for interventions; and develop formal institutional partnerships for periodic assessments of these systems. Volume 4 presents the results of the first global indicator-based, comparative assessment of large marine ecosystems. An assessment of the Western Pacific Warm Pool, based on a sub-set of the indicators, is included.