Computer models designed to simulate the fate and transport of pesticide residues in surface runoff and water that recharges shallow aquifers are used at several stages in the regulation of pesticides. Models can be a powerful tool to identify environments in which pesticides are more mobile and are more likely to impact ground-water quality. Models can also be used to highlight pesticides that can contaminate surface or ground water at an early stage of the registration process. At present, models are utilized primarily as comparative tools to identify which compounds should be examined on a priority basis. Results may be used to calibrate numerical models and estimate the long term impact on the environment from the use of a particular product or its alternatives. To date, field monitoring studies have been required for over 40 chemicals by the Office of Pesticide Programs (OPP) to evaluate the leaching potential of pesticides under realistic usage conditions. As the capabilities of models improve and linkages are developed between databases and the models, computer simulation models will play an increasingly important role in the Environmental Protection Agency's (EPA's) efforts to manage pesticide use to prevent adverse effects on the environment.