We think of the ocean as being driven by a variety of forces: tides, wind, surface buoyancy fluxes associated with heating, cooling, evaporation, precipitation and ice formation, and geothermal heating. Apart from tidal forces, which act throughout the ocean, geothermal plumes that rise far into the ocean interior, and locations of deep convection, these driving forces communicate their influence to the ocean via boundary layers that are thin compared to the ocean depth. In a sense, therefore, the behavior of the ocean is controlled by what happens in these boundary layers. The response of the boundary layers to forcing sets boundary conditions of fluid injection or removal that force a response in the rest of the ocean. We are familiar, for example, with the role of “Ekman suction” as the means by which the wind drives ocean circulation.