Present-day knowledge of the biological effects of ionizing radiation are reviewed together with available information on current levels of exposure. Although our experience has increased over the last few years, it is still not possible to answer the question whether low doses of radiation do in fact produce any harmful effects. On the other hand, the maximum risk of such effects occurring can be assessed, and it is evident that the number of persons affected will not be sufficiently high to be statistically detectable in any exposed group unless the dose substantially exceeds the limits at present recommended as permissible. Special emphasis is given to a survey of the data which are usually taken as the basis for assessment of the risk of radiation-induced somatic effects.The review of present radiation levels includes medical and occupational exposure as well as environmental contamination. The mechanism of fall-out of fission products from nuclear test explosions and the subsequent transport of these products through the food-chains to the human body is now essentially better understood than was the case a few years ago.