In this paper I shall, as a biologist, look specifically at the basis for current maximum permissible exposure levels to radiation and radioisotopes. The public is singularly unaware of the fact that scientists know a great deal about the biological effects of ionizing radiation, and of radioisotopes in man, in animals, and to a lesser extent in the biosphere. True, there are wide gaps in knowledge in this field, as in all of biomedical science. Sci­entists tend to emphasize the gaps and surround their statements with qualifying phrases. Nevertheless, the gaps in our knowledge of radiation can be recognized partly because we have enough fabric to see that there are holes in it, whereas in some other areas of interest to those concerned with the pollution of the environment, there is hardly enough fabric yet to see whether or not there are any holes. I hasten to say that this contrast does not mean that we should relax vigilance toward radiation health hazards. But it does bespeak the relatively high validity of our standards from the standpoint of the underlying biology. We are not groping with the unknown or mysterious to the extent that one might gather from the public clamor and the multiplication of safety factors in radiation standards.