ABSTRACTThere have been two predominate approaches to the modeling of the Great Lakes water system: physical models and mathematical models. The physical models have been of individual lakes whereas the mathematical models have varied from models of individual processes such as evaporation occurring in one portion of one lake to models which include all water quantity components for all five Great Lakes.The assumptions and limitations of the two approaches are presented along with the kinds of results to be expected from each type of modeling. Examples of previous modeling efforts are given to illustrate these assumptions, limitations, and results. Other areas requiring further research are outlined.