Healthy marine and freshwater ecosystems are critical to sustaining life on earth. Water— both freshwater and the oceans—supports and connects ecosystem functions, human health, and key economic sectors. Yet our aquatic ecosystems face unprecedented threats. Ocean ecosystems are under pressure from climate change, acidification,habitat loss, pollution, excessive fishing, unintended effects of shipping, and seabed mining. Freshwater ecosystems face threats from climate change, urbanization, and increasing demand for food, which are polluting our waters and destroying freshwater habitats. Most large marine and freshwater ecosystems cross political boundaries. Globally, more than 300 watersheds and over 360 aquifers cross the political boundaries of two or more countries and 58 out of the 64 large marine ecosystems globally are shared by two or more countries. Transboundary watersheds cover about half of the Earth’s land surface and are home to about 40% of the global population.