The results of a study of the mechanical responses of the head will be presented. Driving point impedance responses of a variety of subhuman primates and unembalmed cadavers have been measured and used to develop a lumped parameter model of the head. The response of these heads to impacts (front and side) have been used to further evaluate model parameters and to develop injury criteria. The model indicates decreasing tolerance with impulse duration for pulses of approximately 10 times the first resonance period; a quasistatic response is indicated for impulses longer than 10 times the first resonance period. The results are compared with other current head injury criteria. Applications of the model to helmet design and restraint systems are suggested. It is intended that this information form part of a crash test device performance specification that will result in an analogue head with human-like responses to be used in the prediction of the injury-reducing potential of head protection devices.