Head is distinguished for his theoretical work in several fields of physics, particularly solid state physics. All his contributions are characterized by insight into the physics of the problem, depth of treatment and originality of thought. Head has shown that, contrary to former belief, the failure of metals by fatigue is not dominated by the initiation of the crack but by the rate of its propagation. The arrangement of defects (on an atomic scale) which govern the crack formation is an n-body problem for whose solution Head has developed several new methods. Predictions of characteristic features, based on anisotropic elasticity, were confirmed experimentally in detail. Recently Head pioneered direct comparison between theoretical and experimental images in the electron microscope, a method now in widespread use. Head has also made contributions to the design of aspheric optical systems, particularly for use at short radio wavelengths.