The actions and biological effects of radiation are diverse and complex, starting from the early stages when radiation energy is deposited into the biological systems and DNA damage is induced, and continuing into the late stages when the actual health effects appear. To understand these processes comprehensively, knowledge from a broad range of fields will be needed, including physics, chemistry, biology and medicine. Ultimate biological consequences of radiation exposure depend not only on

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... doses, dose rates, fractionation, source and quality of radiation, but also on other factors, such as age, lifestyles, environments, tissue oxygen concentration, and genetic backgrounds that determine the repair capacity for DNA damages in each individual. Responses to the DNA damages are activated upon induction of any type of the DNA damages by radiological and non-radiological causes, and play a critical role in preventing clinically detectable adverse effects due to the DNA damages. Hereditary defects in the DNA damage responses lead to high-risk groups for radiation-induced diseases, including cancer, and in such "radiosensitive" populations, even low-dose radiation exposure can cause severe health effects. Further genetic and molecular biological approaches are needed to identify all the high-risk groups for radiation-induced diseases and to develop effective therapies for these conditions based on the radiosensitivity of each individual.