Hydrodynamic processes in the Great Lakes directly affect the chemical, biological and ecological dynamics of the system. Horizontal and vertical transport and mixing influence the distribution of nutrients, contaminants, and biota. This paper discusses some of the conceptual and numerical hydrodynamic models that have been developed for the Great Lakes. It is not intended to serve as a tutorial on lake hydrodynamics but rather as a brief introduction to some of the different types of hydrodynamic models that have been developed for the Great Lakes and as a reference source for researchers who want to investigate the subject further. Several excellent tutorials and reviews on hydrodynamic modeling in the Great Lakes have already been published by Mortimer (1974 and 1984), Csanady (1984), and Boyce et al. (1989). A book on circulation modeling was written by Simons (1980), and a book on coastal hydrodynamics was published by Csanady (1982). The relation of hydrodynamic modeling to biological and chemical processes is covered specifically in reviews by Boyce (1974), Simons (1976c), and Bedford and Abdelrhman (1987).