Across the globe, 276 rivers connect the territories of two or more countries, covering about 50% of the Earth's surface, and 40% of the world's population depends on the freshwater resources from these river basins. Similarly, most of the world's 66 large marine ecosystems (MEs) are shared by more than one country. There are also untold numbers of shared aquifers. Collaboration across sectors and countries is no longer optional: it is crucial to ensure the continued and adequate availability of water for human consumption, and to ensure peace and security.