The chain of reactions, resulting in mutation and cell death, initiated when ionizing radiation interacts with mammalian cells is complex and traverses a time-scale from a fraction of a picosecond to a few hours. Recently, progress has been made in identifying some of the more important pathways and potentially damaging free radical intermediates. Protection can, in principle, result from modification at several steps in these reaction chains, each with its distinctive time-scale. To date we have demonstrated that radio-protection can be effected in mammalian cells by the modification of processes at three quite different times. In this paper, the temporal sequence of radiation-initiated events are reviewed and the potential for or measurement of radioprotection of each is discussed. Such an understanding of cellualr radioprotection has become possible by our improved understanding of cellular radiation mechanisms derived from recent studies of its chemistry [1-4].