Motorcycle and bicycle safety helmets must adhere to specific technical requirements in order to provide protection. Various helmet configurations illustrate success with the application of these principles as well as failure by the neglect of these technical principles. The most critical feature of truly protective headgear is the use of a significant amount of energy absorbing material for the liner of the safety helmet. The most successful safety helmets have been built to standards that incorporate these technical requirements as basic performance in the areas of retention systems and impact attenuation. The qualification of the safety helmet to the basic technical requirements insures protection to the covered integument as well as prevention of a class of “contact” injuries including skull fractures and intracranial injuries. In addition, the qualified safety helmet is shown to prevent or reduce most of a class of “inertial” injuries, but with certain limitations of specific rotational injuries which can not be excluded by any protective device.