Battelle Memorial Institute, Pacific Northwest Laboratory, In the prologue, Dr. Noonan pointed out many of the differences between the format and scope of this symposium and the one held in Oak Ridge in 1953. The interest in this field is indicated by the impressive number of reports published in the intervening years; there are over 1000 literature citations in the papers of this symposium. Of greater importance, however, has been the increased sophistication in approach and in interpretation evident in the studies reported upon in the past 4 days. These studies clearly reflect the conceptual advances that have been made in the past several years. The roles of improved instrumentation and more-definitive approaches to problems of microdosimetry are particularly evident. As we have seen, the measurementof the cross-placental transfer of radionuclides has become highly quantitative, and the available data extend over a wide variety of species, including man. In addition to the determination of radiation doses in experimental animals, suffictent information was available, for a few nuclides, toallow estimation of the radiation dose to the human fetus following exposure of pregnant women, Our progress beyond the descriptive stage is illustrated by the studies on the dynamics of transfer as well as those in which it has been possible to separate the role of the fetus from that of the placenta. The metabolism of inorganic elements by the neonatal organism and the changes which occur during maturation have long been neglected areas of research. Ths amount of quantitative data obtained recently in a variety of species is most impressive. The mechanisms for these age-related differences are also being elucidated through studies on the role of diet and on the binding of radioactive nuclides at the tissue level, 1013 neh eee a name Richland, Washington - MELVIN R, SIKOV ard iatalata Renee eae os anti seattie RE learae EPILOGUE a f : = ie an Lv] i