A general finite element model of a motorcycle crash helmet has been created which incorporates the main features of proprietary designs although of a simplified geometry. The behaviour under impact conditions is investigated, using the Dyna3d computer package, with particular reference to the influence of shell stiffness and polystyrene liner density. Performance is judged in terms of maximum acceleration and the head injury criterion, HIC. That the stiffness of the shell has a major effect on the behaviour has been established, this needing to be low to enhance the value of the helmet in reducing rider head injury in an impact. Furthermore the liner material should have a low density, and hence also a low stiffness, so that the limited space available for its deformation is used to the maximum.