Early education; studies biophysics at Universität Frankfurt and Kaiser-Wilhelm Institut (Friedrich Dessauer, Rievsky); physics training (Erwin Madelung, Meissner); Dessauer's political troubles. Fellowship to Institut Radium (Marie Curie), 1933; building geiger counters (Frédéric Joliot-Curie); life and staff at Institut (Irene Joliot-Curie, Jean Perrin, Hans von Halban, Peter Preiswerk, Lew Kowarski, Rosenblum); Institut's role in development of nuclear physics (P.M.S. Blackett, Giuseppe Occhialini); first nuclear physics conference in Zurich (Paul Scherrer), 1933; London Conference of 1934 (Max Born, Maurice Goldhaber); F. Joliot-Curie thinking about accelerators and about building a cyclotron (Pierre Weiss); Gentner continues gamma ray work (Lise Meitner). Gentner leaves Institut after Curie's death; fellowship at Institute for Medical Research, Kaiser-Wilhelm Institut, Heidelberg (Walther Bothe), 1935-1938; also lectures at Frankfurt on radioactivity, gamma rays, x-rays, and cosmic rays; builds the first Van der Graaf machine in Germany, 1936; first to use gamma rays to look for nuclear photo effect (Fowler, Lauritsen). Travels to United States to study cyclotrons (James Fisk), 1938; spends several months at University of California, Berkeley (E. O. Lawrence, Donald Cooksey); the fission story (Niels Bohr, J. R. Oppenheimer); calibrating ionization chamber and experimental work in fission; life and pre-war politics at Berkeley and Stanford University (Felix Bloch); visits California Institute of Technology (Fowler, Lauritsen, Max Delbrück); travels to Washington, DC (George Gamow, Edward Teller, Fleming, Merle Tuve); and ends tour in New York City (John R. Dunning, Lawrence, Bohr). Returns to Europe; visits John Cockcroft at University of Cambridge. Returns with wife to Germany in April, 1938; plans for Siemens to build cyclotron in Heidelberg canceled. Sent to Paris to interview F. Joliot-Curie on whereabouts of heavy water, July 1940; private meeting afterwards; works in Paris with F. Joliot-Curie on cyclotron, 1940-1942; returns to Heidelberg to build own cyclotron, 1942-1944. Difficulties of re-establishing nuclear physics in Germany after World War II (Cockcroft, Konrad Adenauer); building up new laboratories; CERN, DESY.