The EXPRES expert system is designed to aid regulatory personnel in their assessment of the potential for pesticides to contaminate the soil and shallow groundwater environment. The system consists of one screening assessment model and two mathematical simulation models. This report reviews the theory of pesticide transport and transformation in the unsaturated zone, the models used in the expert system, the development and operation of EXPRES, results from an evaluation of EXPRES using four case studies typical of a pesticide assessment, and scenarios illustrating the application of EXPRES. Appendices include a list of pesticides in the EXPRES data base, sample screens, and lists of EXPRES files. This report serves as a user manual for the expert system.