The use of pesticides in the production of agricultural commodities is widespread. Since nearly one-half of the U.S. population relies on groundwater as their source for drinking water, contamination potential of groundwater, because of pesticide manufacture and use, must be understood. The processes of sorption, biotic and abiotic transformation, and vapor transport have been discussed in previous chapters of the book. Theobjective of the chapter is to integrate the above processes into chemical mass transport models that can be used to forecast environmental exposure. Almost any modeling activity related to groundwater starts with a water flow model, since, for any significant change to take place in the flow field, flow is an essential ingredient. Van der Heijde et al reviewed several hundred groundwater management models from around the world and classified them in a variety of ways. The review included both saturated and unsaturated flow models along with identifying the source and availability of computer codes for the models. Considering the review and several other reviews, the chapter will emphasize chemical transport rather than mass water flow. For completeness, a brief overview of water flow through saturated and unsaturated soils will be included. Decoupling the water and chemical transport is a major assumption in the discussion.