Modeling is increasingly being used as a tool for the evaluation of the environmental fate of pesticides. Sorption, leaching, degradation, and volatilization are some of the processes being integrated through the use of simulation modeling techniques. Several research programs are focusing their attention on such issues (16, 17, 18, 32, 35), with regulatory agencies involved in management of pesticides also taking a modeling approach (3, 7). Because of the extreme complexity of agroecosystems, it is obvious that the use of simulation models will continue to be the most expeditious, reliable, and cost-effective means of integrating the various processes acting upon a pesticide to determine its fate. For example, modeling will help to summarize and interpret efficacy trials and will provide the vehicle for transferring experimental results to unstudied situations, such as the potential environmental fate of an applied herbicide. However, proper development, testing, and responsible use of a modeling approach must be based upon a thorough, comprehensive understanding of interdependent and dynamic natural processes.