"Hormesis" is defined, originally in the field of toxicology, as a phenomenon in which a harmful substance gives stimulating effects to living organisms when the quantity is small. The concept was extended and applied to ionizing radiation, high doses of which are harmful. Although radiation has been thought to be, based on findings in high dose ranges, harmful no matter low the dose is, recent investigation revealed that living organisms possess the ability to respond to low-dose radiation in very sophisticated ways. A good example of such responses is the so-called radiation adaptive response, a process in which acquired radioresistance is induced by low-dose radiation given in advance. The stimulation of certain bioprotective functions, including antioxidative capacity, DNA repair functions, apoptosis, and immune functions are thought to underly the adaptive response. The adaptive response is effective for chromosome induction, acute death, and tumorigenesis induced by high doses of radiation. Radiation hormesis and adaptive response provide a new scope in the risk assessment and medical application of ionizing radiation.