Water is everywhere, the stuff of life, sculptor of the natural environment, helper and headache in a wide range of technologies. The first layer of water molecules at a surface is the structural template that guides the growth of ice, embodies the boundary condition for water transport, and mediates aqueous interfacial chemistry. It thus determines if rain will fall, how fast pollutants migrate in rock and soil, and governs corrosion, catalysis, and countless other processes. © 2010 American Institute of Physics.