Among the developed and potential sources of large public and industrial groundwater supplies in Illinois, the sand and gravel beds of the Pleistocene rank first. These deposits yield most of the groundwater, yield the greatest quantities of water to specific wells, and contain the largest part of the State's undeveloped resources of low-cost groundwater.Groundwater is one of the vital geologic resources of Illinois. Studies by the Illinois State Geological Survey show that about 55 per cent of the State is underlain by one or more aquifers of industrial importance. Twenty-five of Illinois' twenty-six great industrial cities lie in or near those areas of the State where geology is favorable for groundwater development. Seven of the major industrial cities and at least one hundred other communities depend entirely upon Pleistocene sand and gravel beds for groundwater supplies. Wells of highest water yield (2500-3000 G.P.M.) are constructed in valley-train type sand and gravel deposits which are in many places buried by till. The detection and mapping of these deposits involve integration of geologic and geophysical methods of exploration. These sources of groundwater will be of increasing economic significance where industry can move to the undeveloped areas in which low-cost groundwater can be found in abundance.