Oceanic circulation is responsible for the majority of heat transfer within the Earth system and, hence, determines weather and climate for the majority of humankind (Figure 1). Circulation within the ocean is patterned by numerous forces but mostly reflects spatial differences in density and heat that are in turn minimized by a complex system of currents within the major ocean basins. The forces affecting ocean circulation are thermo-haline differences (horizontal and vertical variations in salt concentrations and temperature), surface-wind-generated stress, the Coriolis force (the deflection of moving bodies across the Earth surface due to its rotation), and underwater topography. This entry will discuss the resulting circulation pattern below, which demonstrates the spatial relations between centers of high and low pressure, the gyres these systems create, ...