population, resource demands, and the environmental consequences of our own success as a species. Meeting these planetwide challenges will require a major advance in our ability to understand fluids in and on Earth, manipulate them, and monitor their where-abouts and effects. These challenges are being met by new experimental tools that can illuminate what happens at the microscopic scale on mineral surfaces, new geochemical and geophysical field techniques, and airborne and spaceborne sensors that offer an unprecedented view of how water and other fluids are shaping our planet. The ultimate objective of this research is robust mathematical models that can simulate natural fluid-bearing systems and predict far into the future how they will behave and change. Only by building and skillfully using such models will we be able to make informed decisions about the land and resources that support humankind and all life on Earth.