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Ground-water reservoirs and the overlying unsaturated zone-collectively, the "subsurface"-have an enormous capacity to supply water to wells and useful plants, to store water to meet future needs for the same purposes, and, under suitable precautions, to accept wastes. This capacity can be exploited on a maximum scale, however, only on the basis of information one or more orders of magnitude greater than that available at present on the distribution, recoverability, and replenishability of subsurface water. Because usable water must be made available, and waste water must be disposed of, at costs of only a cent or a few cents per cubic meter, there is a critical need for research to devise methods of accomplishing these water-management tasks at reasonable cost. Among the chief target areas for research in subsurface hydrology are permeability distribution, including vertical permeability; prediction of the departure of the storage coefficient from the theoretically "instantaneous" property assumed in flow equations; theory of unsaturated flow based on fundamental soil characteristics that can be measured practicably; geochemical relations including the effects of injecting water of one composition into zones occupied by waters of different composition, generation of acid mine water, occurrence of saline water, and salt-fresh-water relations in coastal and other areas; prediction of the fate of wastes injected underground; geophysical techniques both surface and subsurface to extend, at low cost, information obtained by other means; and practical techniques of artificial recharge, especially through wells. ?? 1967.