The objective of this thesis was to develop a systematic approach for development of side impact occupant protection systems, which cover the gamut of all dynamic test modes (FMVSS 214 MDB and Oblique Pole, LINCAP, and IIHS) and dummies. This was achieved in five specific steps. First, based on the comparison of structural responses and their effect on occupant responses between the MDB and FMVSS 214 oblique pole test modes, the temporal sequence of events that takes place in each of these test modes are presented in light of the primary momentum exchanges and the underlying physics. Furthermore, the key factors that affect dummy responses in each of these test modes were identified including vehicle and occupant dynamics. Second, a literature review was conducted to identify the gaps in the current state-of-art test methodologies for simulation of full-vehicle occupant responses at a sub-system level in the lab, for development of countermeasures/strategies. Third, to overcome the existing gaps in current test methodologies, two new sub-system sled test methodologies for simulation of dummy responses in MDB and Pole test modes were developed and validated with full-vehicle tests. The development of side occupant countermeasures was done in two phases – development of performance targets for countermeasures that are compatible with both SID-IIs and ES2-re dummies, and evaluation of the countermeasures using component test methodology. In the fourth step, the impact responses of different body regions (shoulder, thorax, abdomen and pelvis/leg) of the ES-2re and SID-IIs dummies have been determined under various initial test conditions by taking effects of velocity, shoulder engagement, initial arm position of the dummy, and oblique loading of thorax into consideration. A new test methodology using rigid flat wall with an initial velocity condition impacting a stationary dummy was developed. The input force was measured using five load plates of the rigid wall, which were aligned with their respective body regions. Cause-and-effect relationship between input loading conditions and dummy responses has been established using linear regression analysis. Further, several correlations that indicate the underlying physics of internal load transfer between different body regions of the dummy have also been identified. Based on the results of the rigid flat wall testing with ES-2re and SID-IIs dummies under similar test conditions, transfer functions were developed between the two dummies. These transfer functions allowed comparison of responses between the ES-2re and SID-IIs dummies in different test modes (IIHS, FMVSS 214/LINCAP, and EuroNCAP), for comparable impact severity. The results of this analysis can be readily applied to assess performance trade-offs, if any, and in the initial target setting for performance of countermeasures (SAB, door trims, EA foams, etc.) that are compatible with both dummies. Finally, for the evaluation of countermeasures, several component test methodologies have been developed and validated for door trims and restraint systems.