Some of the risks and causes of injury to children associated with being unrestrained in car collisions are outlined, based on a sample of high energy crashes. Data from a questionnaire survey of a separate sample of 663 children who were wearing some type of restraint system in collisions is presented. For unrestrainedchildren the results from-these studies show that head injuries are very important, and partial ejection is a frequent mechanism of injury. For restrained children head injuries are the most common type of injury and intrusion and lateral loading are dominant causal factors. There is no evidence of severe neck injuries for restrained children nor any suggestion that deceleration forces without specific contacts are generating major injuries. Intrusion, seat movement and seat damage are the main factors which prejudice the safe ride-down envelope. Some consequences of these data are reviewed in relation to the problems of the dynamic modelling of young car occupants.