Since medical therapy contributes significantly to the population burden of ionizing radiation (Figure 12.1), it is not surprising that workers who utilize sources of ionizing radiation, particu- larly radiologists, will show some excess risk for neoplasia. In fact, the first cancer that was attributed to the effects of ionizing radiation was on the hand of a radiologist, reported in 1902, and leukemia was first associated with chronic exposure to ionizing radiation by radiologists (cf. Boice et al., 1996). The incidence of leukemia in radiologists in the first half of this century was three to four times that in the general population, and skin cancer was also excessive in this group. However, since the middle of this century, such an association has dramatically de- creased, in all likelihood because of the greater control and protection from sources of ionizing radiation used in medical practice. In part because of increased public concern, many studies have been carried out on workers at various nuclear installations and shipyards involved in the building of nuclear-powered vessels. In general, these studies have not indicated any significant risk of cancer in such installations in the United States (Boice et al., 1996). Even after accidental emissions, as at the Three Mile Island Nuclear Plant, subsequent follow-up has not indicated significant influence of this accident on cancer risk in the area (Hatch et al., 1990). However, in the accident occurring in the Chernobyl Nuclear Power Plant in the Soviet Union, excess cases of leukemias and thyroid cancers had already been diagnosed and verified by 1997 (Ivanov et al., 1997). Thus, an accident at a nuclear installation does carry a potential for significant risk of exposure and subsequent carcinogenesis. However, careful maintenance and protective and safety measures have essentially eliminated the environmental risk of ionizing radiation from these sources.