The risks, real and supposed, of the use of radiation in medical practice as perceived by patients, their families, and the general public have a substantial impact on medical practice. Attempts to quantify low-level radiation effects involve much conjecture and supposition. This causes considerable uncertainty in results. Conflicting perceptions follow from this. Radiation exposure limits have been established, and changes in these limits have been made on the basis of the best available judgment at the time. Dose limitations have been lowered over the years. The perceived risk that the public will accept for a given benefit, in part, determines what is permissible. The radiologist must be well informed about radiation effects and must effectively communicate this risk information so that the public will be well informed.