This paper explains the need to plan systematically for the optimal use of groundwater resources in Japan, and describes some of the elements of such a plan. Groundwater extraction has recently been reduced in Japan's largest cities, but major problems, especially of land subsidence and salt intrusion into coastal aquifiers, remain. Three of the worst affected areas are identified. Systems for observing groundwater are described, and the present status of empirical studies and techniques for the simulation of groundwater behaviour are evaluated. Finally, the most effective means of preserving groundwater and preventing subsidence are enumerated.