A series of mathematical models of the interaction between an occupant and the interior of a vehicle is presented. The following parameter studies using an eight-mass, two-dimensional model are discussed: belt material properties, belt slack, belt geometric configuration, and comparison of seats with and without headrests in rear impact. In addition, it is demonstrated by example that simple mathematical models can perform a valuable service in laying the groundwork for more sophisticated analytical and experimental work as well as yielding short term results. Finally, three-dimensional models are discussed. It is shown that a three-mass, three-dimensional model is a logical extension of current simulation efforts in order to provide insight into occupant response in oblique and lateral impact as well as nonsymmetric restraint systems.