In recent years, the biological effects of repeated small doses of ionizing radiation have been the subject of considerable study. Scientific investigation has shown that when certain critical organs are irradiated, changes may occur which impair organ function and alter organ morphology. Irradiation of the reproductive system, for example, may cause mutations in succeeding generations, and irradiation of the hematopoietic system may lead, after the lapse of a few years, to leukemia. Similar qualitative relationships exist between radiant exposure of the thyroid gland and thyroid cancer, between irradiation of the orbit and cataract, and between irradiation of bone and bone sarcoma. In addition to the foregoing, it is known that the life span of an organism may be reduced by radiation exposure. The precise mechanism of this effect, however, is poorly understood. Although much has been written about the biological effects of radiation, it is noteworthy that current knowledge is more qualitative than quantitative. The reasons for this are not difficult to find. The investigation of dose-effects relationships in man is extremely difficult. For small doses of radiation, many years must elapse between the time of irradiation and the time when the effects are first detectable. Also, the effects induced by radiation may be indistinguishable from those due to spontaneous causes; the leukemia produced by radiation, for example, has the same characteristics as the leukemia which develops spontaneously. Finally, since the radiation effects are generally deleterious, one is not free to study them by well controlled human experimentation. The difficulties for the quantitative determination of dose-effects relationships in man may not, however, be insurmountable. Together, they constitute the type of problem which often may be effectively solved by the technics of epidemiology. Through epidemiological investigation it is frequently possible to gain considerable understanding, both quantitative and qualitative, of the effects of a particular toxic agent if a sufficiently large number of individuals, subjected during their normal lives to known doses of that agent, exhibit well defined biological changes. In the application of epidemiological methods to the field of radiation it is necessary that a substantial population be exposed to known quantities of radiation and that the members of that population be followed for a sufficient period to detect any biological changes which may occur. Since, as pointed out above, the biological changes due to radiation are similar to those arising spontaneously, it is necessary to study a comparable population which is not exposed. Furthermore, for each biological effect under study, one must know not only the radiant exposure levels prevailing about each individual but also, since a given effect is often the result of the exposure of a particular critical organ, the dose received by that organ.