The paper describes a remedial action assessment methodology for uncontrolled hazardous waste sites. The model-based methodology consists of three key elements: a set of sophisticated models (one for each hydrologic zone), a data base structure and associated programs to transmit information between models and link the models together, and a set of instructions (referred to as 'unit operation modules') for modifying and using the models to simulate the effects of alternative remedial actions. A system of independent, but linked models is developed because such a structure allows (1) use of one or more models independently where modeling of all three zones is not needed and (2) increased user understanding and confidence because each model can be implemented and verified separately. Such a structure requires a model linkage technique which provides transfer of information between models at their boundaries (e.g., the water table boundary between the unsaturated and saturated groundwater zones). Given the diversity of hazardous waste site conditions and potential remedial actions, guidance on the proper application of the model system to remedial action assessment is essential. This is provided through the unit operation modules. The remainder of this paper discusses the approach used to select the specific numerical models, the design of the model system, and the unit operation modules. The paper is for presentation at the Fourth Management of Uncontrolled Hazardous Waste Sites Conference, October 1983, Washington, D. C.