A car-to-car side impact model was developed using the Cal3D occupant sitrtulation program and validated against test data for the vehicle and the occupant kinematics. Compared to the existing side impact vehicle/occupant models in the literature, this model has a more detailed vehicle side structure representation and a two-segment thorax featuring the rib cage and the spine. The former gives a more realistic impact response for the door and the latter provides the necessary kinematic information for evaluating both the acceleration-based thoracic injury criteria, such as Thoracic Trauma Index (TTI), and deformation-based criteria, such as Viscous Criterion (VC). The parameter variation study indicated that for the specific vehicle modeled, a stiffer side structure and more spacing between the occupant and the door would reduce injury severity indices. The use of padding material between the occupant and the door inner panel resulted in a recluction in the occupant acceleration response but, because of increased duration of occupant contact, an increase in the deformation response. These predictions were confirmed by a recent side impact test series conducted by the MVMA. The simulation results also indicated that the particular selection of the padding material is important in achieving the lowest possible occupant acceleration response.