A model is defined as a representation of the underlying structure of a process or system. Generally, the model has a simple or manipulable structure relative to the system it represents. Thus, the model can provide a clear understanding of the relationships between system components. Model evaluation is necessary because for many models, the ultimate decisionmaker is far removed from the modeling process. Model evaluation criteria include validating the model's theoretical basis, data, and operational dimensions; verifying the computer model; and assessing the model's updating and review capabilities. At present, model evaluation techniques remain more of an art than an established methodology. Four diagrams, about 40 references, and a model documentation checklist are supplied.