The central problem of paleoceanography is the history of the circulation of the ocean. Although speculation about ancient oceanic circulation goes back to the past century, the field of paleoceanography was founded in the 1950s as oxygen-isotope studies suggested that oceanic deep waters were warmer in the past than they are today. Extensive coring of deep-sea sediments by numerous expeditions after World War II was followed by the ocean drilling programs, providing a rich data base. Paleoceanographic interpretations have tried to explain the most obvious changes in sea-floor sediments and their contained fossils: changing paleotemperatures indicated by oxygen isotopes, fluctuations in the calcium carbonate compensation depth, accumulations of organic carbon-rich sediments, and the unexpected abundance of hiatuses in a setting which had been thought to be the ultimate sedimentary sink. The result has been the intriguing discovery that although the positions and circulation of the major surface gyres is generally stable, the deep circulation of the ocean may reverse on a variety of time scales. It has been suggested that formation of North Atlantic Deep Water, which causes the uneven distribution of nutrients, alkalinity, and oxygen in the deep sea today, may have been replaced by formation of North Pacific Deep Water during the last deglaciation, reversing the concentration gradients of nutrients, alkalinity, and oxygen. On a longer time scale, the present general circulation, which is dominated by production of oxygen-rich cold deep water in the subpolar regions today, may have replaced a pre-Oligocene general circulation in which warm, saline, oxygen-poor deep waters were formed in warm seas in the arid zones. Paleoceanography is still in its infancy; many new clues to the history of the ocean are being discovered, and many new ideas about conditions in the past are being developed. The beginning of the next century should see continuing rapid growth and maturation in this exciting new field.