An extensive study was conducted studying the optimization of seat belt and airbag systems for simultaneous use during an accident. It is an objective of this study to develop an understanding of restraint system interaction and the compromises required for the individual systems to optimize the total response. The study entailed over one hundred full scale dynamic tests controlling seat belt and airbag attributes carefully and varying these attributes methodically. The study shows that significant reductions in occupant injury criteria are possible by tuning the seat belt characteristics predominantly to approximate an ideal belted restraint system. These so called “Constant Force Restraint Systems” can be further expanded to include new levels of pretensioner performance thereby allowing significant injury criteria reduction without increased head or chest excursions. These changes to the belt system also affect the response of the airbag restraint because of the significant changes in the occupant excursion time history. Improvements in the belt system are found in three major categories; initial system stiffness, new levels of pretensioner performance, and careful tuning of the torso harness load limiting response. Further system improvements can be obtained by carefully matching the load limiting response of the torso restraint to the ride down characteristics of the airbag thereby attaining a closer approximation of ideal restraint. Such systems however will require tunability based on the operating environment at the time of the accident.