Although the existence of a global hydrologic cycle has long been recognized, researchers are only now uncovering a vastly wider web of connectivities that binds together the flow of water on a global scale. The connectivities are physical (e.g. upstream storages of water cause large scale changes in the residence time of surface water), economic (e.g. water is embedded in food and other products and traded internationally), and even institutional (e.g. decisions about trade of water technology have a global impact). This new awareness of connectivities has spawned the concept of the 'global water system'. New findings have also made it clear that the global water system is undergoing unprecedented, large scale, and poorly understood changes which are increasing the vulnerability of ecosystems and society. The international community of water researchers and managers can respond to these risks by taking on several 'grand challenges' including: investigating the feasibility (and desirability) of global water governance, improving the global assessment of water resources, developing global early warning systems for floods and droughts, and initiating a new global initiative for benchmarking the loss of aquatic species. These and similar actions would bring a new and needed global perspective to water research and management.