Injury and disability associated with head (brain), neck (spinal cord) and facial injury account for 61.7% of the total societal Harm in the most recent estimate of motor-vehicle related crash injuries. This paper discusses the need for accurate information on translational and rotational acceleration of the head as the first step in critiquing the Head Injury Criterion (HIC) and other injury predictive methods, and developing a fuller understanding of brain and spinal cord injury mechanisms. A measurement system has been developed using linear accelerometers to accurately determine the 3D translational and rotational acceleration of the Hybrid III dummy head. Our concept has been to use the conventional triaxial accelerometer in the dummy's head to assess translational acceleration, and three rows of in-line linear accelerometers and a least squares analysis to compute statistical best-fits for the rotational acceleration about three orthogonal axes. This process provides a standard error or confidence interval of the computed rotational accelerations as a function of time. With accurate rigid body acceleration computed, the inertial response of the head can be subtracted from the neck reaction force to obtain the applied force on the head or face, or determine the lack of head contact. This paper describes a design for an in-line accelerometer package, the hardware, instrumentation and computer software to determine head dynamics, the validation of the measurement analysis packages, and a demonstration of the characteristics of head dynamics. The demonstration includes rotational and translational acceleration responses and head impact forces obtained in a variety of Hyge sled and mini-sled tests. The resulting head dynamic responses demonstrate high levels and various combinations of rotational (typical values of 4,000-6,000 rad/s2 with peaks over 20,000 rad/s2) and translational (typical values of 100-150 G with peaks of 250 G, and HIC values of 200-2500) acceleration during direct impact and indirect loading of the head.