The waters of the earth are gathered in shallow, irregular, interconnecting basins. Heated by the sun and driven by the wind, the oceans circulate endlessly. The general circulation of the ocean is the persistent pattern of this flow on the scale of the basins. It is the heart and soul of physical oceanography, and the explanation and prediction of the flow from the principles of fluid dynamics is the chief goal of the physical oceanographer. Along with the pattern of the motion, the associated fields of pressure, temperature, salinity, and density are also necessary components of a complete theory for the ocean circulation since they are dynamically linked to the motion of the oceans. The physical problem posed by the general circulation of the oceans is a difficult one both experimentally and theoretically. From the point of view of theoretical fluid mechanics the difficulty springs fundamentally from the recirculating character of the circulation. The fluid is gathered into a single, though highly contorted basin.