In the maintenance and development of sound standards of radiation protection, many types of enquiry are important. These range from the purely technical examination of the ways in which exposure may occur or radionuclides may enter the body under existing or new conditions of occupational activity, to the most fundamental studies of the interaction of high energy particulate radiation with matter or the metabolic localisation and kinetics of unfamiliar radionuclides. One of the most important, and most basic, problems, however, is to establish a quantitative estimate for the frequency with which various types of injury would be induced in man following exposure to low doses of radia tion delivered at low dose rates. No sound limits can be proposed for appropriately safe levels of occupational or population exposure unless the associated hazard-or its maximum likely value-can be at least approximately assessed. It is one of the most important tasks of those concerned with radiation protection that these criteria should be kept under constant review in the light of developing knowledge, and that the fields in which further information is needed should be defined and described.