Water can be tricky. With too little, crops die, industries move away, power plants fail, ecosystems suffer and people go thirsty. With too much, floods ruin infrastructure, destroy crops, spread waterborne diseases, and disrupt flows of clean water, wastewater, power and transportation. We want water at the right time and in the right place because moving and storing water require effort. We also want it at the right quality and the right temperature. If we had unlimited clean energy at our disposal, we could desalinate the ocean, providing enough potable water for everyone, everywhere. Energy provides a constraining factor on the world’s management of water issues; likewise, water is a constraining factor on the world’s energy supply. This interdependence is the energy-water nexus and the way we manage the delicate relationship between energy and water will have major implications for the future of both critical resources.