This paper describes wheel-terrain contact modeling in the ROAMS physics-based simulator for planetary surface exploration rover vehicles. ROAMS models a wide range of rover systems and provides a user interface to a simulated rover. The wheel-terrain interaction is of primary interest during rover motion over rough terrain. The wheel-terrain contact model must provide physically realistic behavior without being too computationally intensive for real-time simulation. Physically realistic behavior may be defined using empirical data and ROAMS provides mechanisms for tuning the contact model parameters to match this data. In cases where empirical data is non-existent or there are large amounts of uncertainty, ROAMS can be used to extrapolate a range of behaviors based on physical parameters. The ROAMS simulator can be used in stand-alone mode, for closed-loop simulation with on-board software or for operator-in-the-loop simulations.