Future deep space missions and International Space Station exposures will be dominated by the high charge and high energy (HZE) ions of the Galactic Cosmic Rays (GCR). Such ions deposit energy in tissue mainly by collision with orbital electrons which defines the lateral extent of the energy deposit around the ion path. The alpha decay particles and nuclear recoils from fission neutron collisions, in terrestrial exposures, are of low energy and the low energy of the recoil atomic electrons in tissue limits the lateral distribution of the associated energy deposit leading to assumed LET dependent risk models (for example, Q(LET) in conventional protection practice). In distinction, the HZE ions produce broad tracks and the risk models depend on the lateral extent as well as the LET. A few mammalian systems have been extensively tested over a broad range of ion types and energies. Such systems are C3H10T1/2, V79, and Harderian gland tumors, which have been described by various track structure dependent risk models. The attenuation of GCR induced biological eff-ects depends strongly on the biological endpoint and risk model used. Optimization of space shielding is then driven by the nature of the appropriate risk model and the transmission characteristics of the given material.