Protection from the hazards of ionizing radiation in the space environment has been identified as critical to human exploration, and is of the utmost importance both for journeying to and living on other planetary bodies. The major radiation hazards for exploration class missions outside of the Earth's magnetosphere are due to protons from solar particle events and to the highly charged, energetic (HZE) particles constituting galactic cosmic rays. The mean free path for nuclear interactions of HZE particles is comparable to shielding and tissue thicknesses present in human interplanetary exploration, resulting in a significant fraction of nuclear reaction products at depth. The energy deposition of HZE particles, on the microscopic scale of cells, is extremely non-uniform. Since the physics and biology of HZE particles have been studied for a much shorter time than that of other types of radiation, current knowledge about the biological effects of space radiation cannot predict astronaut health hazards with acceptable precision. Such predictions are required in order to define acceptable risk levels for space exploration and specify shielding for the lunar base, lunar vehicles, and Mars spacecraft. Attempts to deal with present uncertainties by making worst case assumptions may overestimate the required shielding thickness by as much as a factor of 10 and lead to inordinate vehicle masses. Major sources of these uncertainties will be illustrated. The NASA Life Sciences program to resolve the critical problems posed by ionizing radiation in interplanetary space, which consists of an expanded ground-based research effort and space-based validation using the LifeSat satellite, will be described.