Numerous head impact tests have been carried out by the Biomechanics department of the Highway Safety Research Institute, University of Michigan, Ann Arbor, during the past few years. The purpose of this work is to investigate the head motion during impact loading (the unloading phase is not considered) and especially to determine if the head slips on the impactor or rotates around it. Data from three different types of tests have been analyzed: Front, side and rear impacts on human heads. The data consists of linear and angular accelerations and velocities for the head (considered as a rigid body). These four quantities are some of the most common that have been suggested as primary causes of injury. By using experimental data the position of the instantaneous point of rotation was determined as a function of time during impact loading. It was found that slip occurs between the impactor and the head with very few exceptions. It was also found that the human skull performs plane motion after front or rear impact. This is probably due to the symmetry around the midsagittal plane. In side impact on the human skull it was found that the motion is three-dimensional.