Most catastrophic cervical spinal injuries occur as a result of head impacts in which the head stops and the neck is forced to stop the moving torso. In these situations the neck is sufficiently fragile that injuries have been reported at velocities as low as 3.1 m/s with only a fraction of the mass of the torso loading the cervical spine. Cervical spinal injury occurs in less than 20 ms following head impact, explaining the absence of a relationship between clinically reported head motions and the cervical spinal injury mechanism. In contrast, the forces acting on the spine at the time of injury are able to explain the injury mechanism and form a rational basis for classification of vertebral fractures and dislocations. Fortunately, most head impacts do not result in cervical spine injuries. Analysis of the biomechanical and clinical literature shows that the flexibility of the cervical spine frequently allows the head and neck to flex or extend out of the path of the torso and escape injury. Accordingly, constraints which restrict the motion of the neck can increase the risk for cervical spine injury. These observations serve as a foundation on which injury prevention strategies, including coaching, helmets, and padding, may be evaluated.