A comprehensive three dimensional model of the human head and neck is formulated. This model predicts the center of mass displacements, velocities, and accelerations of the head and neck resulting from contact and/or inertial impact forces. Key anatomical components are incorporated in this model along with a joint stopping mechanism. Known acceleration profiles are inputed to the torso and/or head force time histories are specified. The equations of motion are then derived using d'Alembert's form of Lagrange's Principle and are numerically integrated using a fourth order Runge-Kutta technique. Validation is accomplished by the comparison of responses from (i) direct frontal and occipital impact experiments on human cadavers, and (ii) sled tests conducted on human volunteers.