This paper describes the development and validation of a three- dimensional temperature model for the Great Lakes. This model is keyed to simulating horizontally and temporally varying surface temperature based on an existing 1-D eddy diffusion model. In addition, a 2-D ice cover model is developed to simulate the formation and ablation of ice cover. The ice cover model couples to the water temperature model, forming a spatially distributed thermodynamic model for the Great Lakes. This model can be used to give long-term or short-term simulations for water surface temperature and ice cover. We compare the satellite-derived water surface temperature from NOAA CoastWatch program with the simulated surface temperatures from 1992 to 1995. Observed ice cover data are used to develop and calibrate the ice cover model.