The boundary layer is the lowest 1-2 km of the atmosphere, the region most directly influenced by the exchange of momentum, heat, and water vapor at the earth's surface. Turbulent motions on time scales of an hour or less dominate the flow in this region, transporting atmospheric properties both horizontally and vertically through its depth. The mean properties of the flow in this layer—the wind speed, temperature, and humidity—experience their sharpest gradients in the first 50-100 m, appropriately called the surface layer. Turbulent exchange in this shallow layer controls the exchange of heat, mass, and momentum at the surface and thereby the state of the whole boundary layer. It is hardly surprising we should have a lively curiosity about this region.