The consideration of groundwater in water-resource planning frequently has been neglected because many planners believed that groundwater could not be adequately evaluated in terms of availability, quality, cost of development, or effect of development on the surface-water supply. The development of predictive groundwater models now provides the water planner with tools to evaluate these problems. Highly sophisticated digital models can be used in planning the development of groundwater and the conjunctive use of ground- and surface water. About 250 digital models have been used to evaluate groundwater problems: The models are powerful tools for predicting the response of groundwater systems to stresses. They can also clarify the cause and progress of past stresses. With these developments it is now possible to integrate the utilization of groundwater into water-resource planning with a high degree of confidence. ?? 1979.