Current regulations mainly consider maximum headform acceleration as the criterion to evaluate crash helmet effectiveness in terms of shock absorption. From more basic research, however, five general phenomena have been identified to be associated with head protection: the kinetic energy involved in the impact, the maximum load on the head, the load distribution, the rate on onset of loading and the time duration of the impact. Considering the high importance of head injury prevention as a whole and the ongoing discussions on the mechanicms of head injury, it seems worthwile to address helmet optimization on the basis of these 5 phenomena. An experimental study has been performed to address helmet optimization, mainly considering the characteristics of the outer and inner shell. Using the ECE Regulation 22 test set-up, some effects of outer and inner shell characteristics have been studied. This study clearly shows the different deformation models of stiff shelled helmets compared to soft shelled helmets. Helmet performance is not a pure matter of inner shell density, as has been suggested by others. Helmets having a fiber-reinforced plastics (FRP) outer shell absorp energy predominantly by inner shell deformation 'from the inside'. Helmets having polycarbonate (PC) outer shells absorb the energy predominantly 'from the outside'., These different deformation modes imply opposing tendencies for the phenomena studies, especially with respect to load distribution and the rate of onset of loading. Further optimization of helmets can effectively be performed provided all five phenomena are taken into account. This requires further development knowledge on the thresholds for these phenomena and a suitable test method to evaluate the load distribution.