The expert system described here is designed to aid regulatory personnel in their assessment of the potential for pesticides to contaminate groundwater. The expert system, known as EXPRES (EXpert system for Pesticide Regulatory Evaluation Simulations), consists of existing simulation models coupled with a knowledge-based system. The numerical models are used to simulate the transport and transformation of pesticides in the unsaturated zone. The knowledge-based system guides the user through the choice of all the necessary information for characterizing the physical, climatic, hydrogeological, pedological and agricultural settings of typical agricultural regions across Canada required by the pesticide model, as well as aiding the user with the model predictions. EXPRES is designed to be used as a management tool to aid in policy decisions and is not intended for use as a research tool. Thus, its purpose is not to provide insight into the processes that control the fate of pesticides in porous media, but to provide an assessment of the potential hazards and to identify if further study is warranted.