Existing data on factors at work in athletic injuries to the head and neck are reviewed. Measurements have been made of the energies, accelerations, and changes of intracranial pressure involved in cases of concussion and skull fracture. In the cadaver, linear skull fractures can be produced by energies of 4.6 to 6.9 kg.m. These impart an average acceleration of 112 g and increase the intracranial pressure by about 1,450 mm. Hg. Such figures afford a basis for the construction of protective helmets. The thickness of padding required can be computed from the weight and velocity of the injuring object. If the velocity is doubled, the padding has to be 4 times thicker.