This paper reports on the development and use of a three-dimensional mathematical model of vehicle occupant kinematics for the purpose of studying various seat, restraint system, and occupant configurations in a crash environment. This model includes three masses representing the head, torso, and lower limbs. Contacts between the occupant and a large variety of moving interior vehicle contact surfaces are allowed. Decelerations in the three linear directions as well as vehicle pitch, roll, and yaw can be accepted as input to the computer program. Program output can be presented in the form of tabular listings of kinematic and force variables as well as time-sequenced drawings in the form of 16 mm. movies of the occupant configuration within the vehicle and restraint system.A study of the angles and three-dimensional attachment points of various seat belt and shoulder harness configurations is presented including tabulated summaries of the computer-generated data. Comparisons are made between the performance of single diagonal, inverted y-yoke, and double shoulder harness arrangements in both frontal and lateral impact. The paper is concluded by a discussion of how mathematical models of this nature can be used to optimize restraint system performance and an example application of the techniques to rearend impact protection.