The development and application of mathematical models to simulate water quality behavior in ground-water basins is at a very youthful stage. There is still much to be learned and a great deal of experience to be gained. However, in the last few years, models have been developed to simulate conservative and some nonconservative constituents under both saturated and unsaturated soil conditions. In addition, models have been found to provide additional information for calibration of quantity models, to identify major data gaps and deficiencies, to be useful in the design of data collection programs and in the interpretation of collected data, to be valuable to watershed planners and managers through evaluation of alternative regulatory policies, physical facilities and management operational plans, and to point out where future research efforts are needed.