Target theory is one of the essential concepts for understanding radiation biology. Although many complex interpretations of target theory have been developed, its fundamental principle is that ‘inactivation of the target (s) inside an organism by radiation results in the organism’s death’. The number of ‘targets’ and the locations of these ‘targets’ inside an organism are not always clear, although the ‘target’is considered as a unit of biological function. Assuming that when an average one-hit dose (inactivation of one target) per organism is used and one-hit of irradiation results in the organism’s death, then the probability of survival [P (1)= S] is expressed by: