Follow-up studies of persons exposed to medical radiation have long shown that radiation induces cancer in man. This, coupled with increasing exposure from other sources including occupational and environmental radiations, has resulted in greater recognition of the importance of research on radiation-induced carcinogenesis and risk assessment with a view to radiation protection. One of the well-known late effects of radiation is the increased incidence of leukemia that occurred among atomic bomb survivors beginning two or three years after expo sure. A remarkable increase of solid tumors including cancers of the thyroid, breast and lung was also observed 10 to 20 years after exposure. Thus, many pathological, clinical and epidemiological studies have been made on radiation carcinogenesis in atomic bomb survivors by investigators at the Atomic Bomb Casualty Commission (ABCC), now known as the Radiation Effects Research Foundation (RERF), as well as by the staff of universities in Hiroshima and Nagasaki. Some of the mechanisms involved in radia tion carcinogenesis in man and associated modifying factors, such as age at time of ex posure and sex, have been elucidated by these studies. The results obtained are being used by such agencies as the International Commission on Radiation Protection (ICRP) for risk estimations of radiation exposure. This monograph presents the results realized thus far in these epidemiological and The incidence of radiation-induced cancer among atomic bomb pathological studies.