Clinical studies have demonstrated that, with regard to death, the two worst types of head injury are subdural haematoma (SDH) and diffuse axonal injury (DAI). These two have different mechanisms of causation; SDH occurs much more commonly in non-vehicular injuries, especially falls, while DAI is caused, almost exclusively by vehicular mechanisms. The production of these two types of injury in non-impact acceleration models helps to explain these causal differences, but also shows that both injuries share a common mechanical cause, differing only in degree. SDH is due to vascular injury that is caused by relatively short duration angular acceleration loading at high rates of acceleration. These are the circumstances that occur in falls where the head rapidly decelerates because of impact to firm, unyielding surfaces. DAI is also due to angular acceleration of the head, but occurs most readily when the head moves coronally and it only occurs when the acceleration duration is longer and the rate of acceleration lower than conditions that produce SDH. These conditions are met in vehicle occupants where impact to deformable or padded surfaces lengthens the deceleration and decreases its rate. In DAI the principal mechanical damage is to the brain itself (mainly to axons) while in SDH the primary damage occurs to surface blood vessels. Now that models of the two most important types of head injury have been created in the laboratory, it is hoped that a better understanding of their pathophysiology will result in new strategies to affect protection from their occurrence and in improved treatment when they do occur.