Animals that have been exposed to a very low dose of radiation are known to have many physiological benefits. Very low dose of ionizing radiation also induces mechanisms whereby cell or tissue become better fit to cope with subsequent exposures of high doses. This phenomenon of low dose radiation is termed 'adaptive response'. This response has been reported to be true in many biological systems and confirmed by experiments on chromosomal and chromatid aberrations, micronucleus formation, sister chromatid exchange tests, DNA mutation and cell survival study and using many other biological end points, although there are quite a few exceptions. The adaptation induced by low doses of radiation has been attributed to the induction of an efficient chromosome break repair mechanism at molecular and biochemical level. It is also substantiated in whole animal systems. When mice are initially conditioned with very small adapting doses, incidence of a challenging dose induced thymic lymphoma is recorded, with delayed latency and reduced frequency. Similarly, appearance of a transplanted barcl-95 thymic tumor has been delayed when mice are preconditioned with a small dose of radiation. Appearance and development of a tumour following transplantation of in vitro irradiated barcl-95 tumour cells with a small dose of 1 cGy are also delayed and volume of the tumour is reduced. Latency period of radiation-induced leukemia is modified by prior treatment with an adapting dose of radiation. Neoplastic transformation of several human cultured cells is also significantly decreased by prior low dose exposure of radiation compared to non-exposed cells. These results indicate that an earlier exposure to a small dose of radiation also reduces the radiation-induced carcinogenesis. Various aspects of molecular mechanism underlying the radio-adaptation have been explained. However, the mechanism underlying the inhibition of carcinogenesis by low dose radiation is yet to be fully resolved.