How much radiation is too much? J. Samuel Walker examines the evolution, over more than a hundred years, of radiation protection standards and efforts to ensure radiation safety for nuclear workers and for the general public. The risks of radiation - caused by fallout from nuclear bomb testing, exposure from medical or manufacturing procedures, effluents from nuclear power, or radioactivity from other sources - have aroused more sustained controversy and public fear than any other comparable industrial or environmental hazard. Walker clarifies the entire radiation debate, showing that permissible dose levels are a key to the principles and practices that have prevailed in the field of radiation protection since the 1930s, and to their highly charged political and scientific history as well.