Prediction of the movement of contaminants in groundwater systems through the use of models has been given increased emphasis in recent years because of the growing trend toward subsurface disposal of wastes. Prediction is especially critical when nuclear wastes are involved. Contaminant transport models which include the effects of dispersion have been applied to several field situations. However, factors that limit the routine use of these models include the difficulty of determining the field coefficient of dispersion and numerical difficulties encountered when solving the dispersion equation. Regional size models which neglect the effects of dispersion have had limited success because of the scarcity and poor quality of field data. Another difficulty in the development of contaminant transport models is the current lack of knowledge regarding the quantification of chemical reaction terms. This review examines the formulation of contaminant transport models, application to field problems, difficulties involved in obtaining input data, and current status of modeling efforts.