Physics-based modeling and simulation of planetary rovers is an integral part of planning, testing and design of robotic planetary missions such as the up- coming Mars Science Laboratory (MSL) and the extremely successful Mars Exploration Rovers (MER) Spirit and Opportunity. As planetary rover missions grow more challenging with heavier rovers moving at higher speeds over rougher terrain than in the past, modeling the vehicle terrain interactions has become a critical part of the success of model-based testing and operations within the NASA/JPL physics-based ROAMS rover simulator. This paper focusses on the wheel-terrain contact model used in the ROAMS simulator and its validation for the FIDO class rovers. It also presents a brief overview of modeling planetary rovers and terrains within the ROAMS simulator. Since the ROAMS simulator is used in various modes such as stand-alone simulation, closed-loop simulation with on-board software simulation, or for operator-in-the-loop simulations, faster than real time computational performance is an essential requirement. Consequently, the vehicle terrain interaction model is based on a physics approach designed to retain adequate fidelity without compromising performance while providing the ability to vary parameters to model different soil and rover properties.Physics-based modeling and simulation of planetary rovers is an integral part of planning, testing and design of robotic planetary missions such as the up- coming Mars Science Laboratory (MSL) and the extremely successful Mars Exploration Rovers (MER) Spirit and Opportunity. As planetary rover missions grow more challenging with heavier rovers moving at higher speeds over rougher terrain than in the past, modeling the vehicle terrain interactions has become a critical part of the success of model-based testing and operations within the NASA/JPL physics-based ROAMS rover simulator. This paper focusses on the wheel-terrain contact model used in the ROAMS simulator and its validation for the FIDO class rovers. It also presents a brief overview of modeling planetary rovers and terrains within the ROAMS simulator. Since the ROAMS simulator is used in various modes such as stand-alone simulation, closed-loop simulation with on-board software simulation, or for operator-in-the-loop simulations, faster than real time computational performance is an essential requirement. Consequently, the vehicle terrain interaction model is based on a physics approach designed to retain adequate fidelity without compromising performance while providing the ability to vary parameters to model different soil and rover properties.