Life in the atomic era has brought with it increased exposure to ionizing radiation. Radiobiology is concerned with the study of the effects of this physical agent. The clinical application of the facts derived experimentally may apply not only to atomic bomb catastrophes, but also to exposures to x-rays in daily medical practice. Ionizing radiation may alter a key physiologic process thus producing an aberrant cellular differentiation. An undetectable acute change may set the stage for the appearance after a long latent period of (a) congenital anomalies or infertility; (b) carcinogenesis; (c) premature aging and degenerative changes. Minimal dose levels for producing these changes in humans is not known. However, experimental work to date indicates that the young recipient of x-rays is more apt to develop late sequelae than is the adult. In addition, his longer life span makes him a more likely subject for such changes. Because of unavoidable exposure to irradiation in industry, medicine and warfare, it is suggested that elective pediatric roentgen procedures be limited as much as possible. Roentgen exposure diaries for patients are recommended.