WCRP’s Grand Science Challenges represent major areas of scientific research, modeling, analysis, and observations for WCRP and its four core projects, including GEWEX. The Grand Challenges are promoted through workshops, conferences, and planning meetings, allowing the community to identify high priority research that relies on international collaboration and results in information that decision makers can put to good use. GEWEX is the lead project for two Challenges, Changes in Water Availability and Understanding and Predicting Weather and Climate Extremes. It contributes in multiple ways to the others, especially the Clouds, Circulation, and Climate Sensitivity Challenge. All of the Grand Science Challenges can be found here, along with explanations of the issues behind them.