In many areas along the sea‐coasts of the world the water‐supply for human use is derived largely, and in some areas wholly, from underground sources. Because of the proximity to the ocean in these areas, in some respects the geologic and hydrologic conditions that govern the occurrence and movement of ground‐water are quite different from those in inland areas, and to properly interpret them the ground‐water hydrologist must avail himself of facts developed by the oceanographer.These conditions are encountered in a marginal zone that in different localities may extend only a few feet or many miles inland from the coast, and presumably for greater or less distances out beneath the ocean. These special conditions have been recognized and extensively studied for many years, for example, in the lowland‐area of Holland that lies between the Zuider Zee and the North Sea, and on Long Island, New York. Within recent years more or less attention has been given to these conditions in many of the coastal states of the United States, and in the Hawaiian Islands and Cuba. It is the purpose of this statement to indicate some of the problems of groundwater hydrology that are related to oce