Today, the ocean is characterized by pools of warm tropical-subtropical water bounded poleward and at depth by cold water. In the tropics and subtropics, the warm waters are floored at depth by the thermocline-pycnocline, which crops out on the ocean surface between the subtropical and polar frontal systems that form the poleward boundary. It is along and between the frontal systems that the thermocline waters enter the ocean interior. These frontal systems form beneath the maxima of the zonal component of the westerly winds. Today, the location of the westerly winds is stabilized by the persistent high-pressure systems at the polar regions produced by the ice cover of the Antarctic and sea-ice cover of the Arctic. The paleobiogeographic distribution of plankton fossils indicates that, prior to the Oligocene, the subtropical and polar frontal systems were not persistent features. Recent climate model experiments show that without perennial ice cover in the polar regions a seasonal alternation between high and low atmospheric pressure systems can occur. These seasonal alternations would force major changes in the location and strength of the westerly winds, preventing the development of the well-defined frontal systems that characterize the Earth today. Without the subtropical and polar frontal systems, the thermocline would be less well developed and the pycnocline could be dominated by salinity differences. Evidence from ocean drilling suggests that the glaciation of East Antarctica began at the Eocene-Oligocene boundary, but took time to spread over the entire continent. The presence of calcareous nannoplankton in the Arctic basin prior to the Oligocene and their absence thereafter suggests that the ice cover of the Arctic Ocean also developed at the Eocene-Oligocene boundary. Both events appear to be related to the development of the modern oceanic structure, but it remains uncertain whether the ocean changed in response to the development of ice covered polar regions or vice versa.