A fundamental model for numerically simulating the in situ thermal recovery of oil from a tar sand resource has been developed. The model accounts for the three-phase flow of oil, water, and gas. Mass transfer among the phases and components is dictated by liquid and vapor equilibrium, reaction kinetics, reservoir properties, and processing conditions. Energy is transferred by the mechanisms of conduction, convection, reaction kinetics, and by revaporization and condensation. The spllltion technique employed in the model is based on a method using the Newton-Raphson iteration scheme with a lariable time-step algorithms. Results are compared with results of another numerical simulator as well as with results from two forward combustion experiments conducted at WRI. These initial comparisons have shown good agreement and have identified areas for further improvement. A comparison of the model's results with the results from the two forward combustion experiments indicates the need for a better understanding of the mechanisms of a pyrolysis and coke combustion. In addition, the predictive capabilities of the model should be enhanced by accounting for the production and depletion of CO, H2, H20, and CO2 by way of the steam char, the secondary char, and the water gas shift reactions.