The radiosensitivity of the embryo and fetus is a matter of great practical importance to radiologists, gynecologists, and obstetricians. It is unfortunate, therefore, that the literature on this subject is diffuse and contains numerous apparently contradictory reports and opinions. Cases of grave radiation injury to the embryo or fetus have been summarized and discussed by Goldstein and Murphy in 1929, Gauss (quoting a thesis by Kraemer) in 1930, Flaskamp in 1930, and Miller, Corscaden and Harrar in 1936. Since the time of these reviews, more cases have been described by Murphy, Shirlock and Doll (1942), Jones and Neill (1944), and others. On the other hand, there are scattered reports of normal births following heavy doses of radiation: Robinson (1927) has collected 23 such cases from the earlier literature, Lacomme (1931) reports 2 and Hobbs (1950) 1. Results from recent extensive animal experiments fall into an easily comprehensible over-all picture which dispels some of the apparent contradictions in the human data. Conclusions from this more unified view will be discussed here and will form the basis for practical recommendations which, in the authors' opinion, may protect the well-being of many unborn infants in this age of increasing use of radiation. The earliest animal work was almost exclusively concerned with the production of abortions and resorptions. Somewhat later investigators—e.g., Pagenstecher, working with rabbits (1916), and Hanson (1923), Bagg (1922), Murphy and de Renyi, working with rats (1930)—reported a variety of scattered abnormalities. Although this work was valuable in showing that malformations could be produced by prenatal irradiation, it lacked the important feature of accurate timing of the stage of gestation at which irradiation was done. Job, Leibold, and Fitzmaurice (1935) were the first who attempted to determine whether there were critical periods in development, either for the mammalian embryo as a whole or for certain of its organs or systems. Although a good part of the work was exploratory, they were able to correlate irradiation of rat embryos on the ninth, tenth, and eleventh days postconception with the later appearance of hydrocephalus, eye abnormalities, and jaw deformities, respectively. Kaven (1938), working on another species, the mouse, lengthened the list of abnormalities for which critical periods were probably demonstrable. He found brain hernia and “extrakranielle Dysencephalie” produced exclusively by irradiation on the seventh and eighth days, tail malformations resulting from irradiation between the ninth and thirteenth days, and indications of the existence of critical periods for several other abnormalities. Three years ago we embarked on an extended series of experiments, the first part of which was a careful mapping of periods critical for the induction of abnormalities by radiation in the mouse (Russell, 1950).