Early interest in science; training in physics and graduate work at Johns Hopkins University, courses, books, seminars, informal atmosphere, 1927-1933; summer work in spectroscopy laboratory at National Bureau of Standards, 1929; interest in elementary particles, attractions of nuclear physics, 1933; postdoctoral work with Gregory Breit on pair theory, New York University, 1933-1934; potential well model and its application to experimental results, 1933-1935; studies at Niels Bohr's Institute, Copenhagen 1934-1935; discussions in Copenhagen on electrodynamics; role of experimental results in formulation of Bohr's concept of compound nucleus, 1935; reasons for initial skeptical reaction to compound nucleus model; significance of Bethe-Bacher-Livingston review articles in Reviews of Modern Physics, 1936-1937; development of compound nucleus ideas after 1935; use of compound nucleus and Breit-Wigner formula as guide for experiment; development of collective model; reaction to Hideki Yukawa's 1935 meson theory and subsequent new cosmic ray particles, late 1930s; establishment of cosmic ray laboratory at Princeton University 1945; work considered personally satisfying; scattering matrix and other work at University of North Carolina, 1935-1938; close relationship of physics and mathematics departments at Princeton, 1938- ; collaboration with Bohr on theory of fission, Princeton 1939; relation of fission physics to other aspects of nuclear physics; plans of physicists for postwar work. Also prominently mentioned are: Joseph Sweetman Ames, Robert Fox Bacher, Hans Albrecht Bethe, Max Delbr ck, Paul Ehrenfest, Gray, Werner Heisenberg, Karl Ferdinand Herzfeld, Hendrik Anthony Kramers, Maria Ge̲ppert Mayer, Mernihan, Plesset, Tarrant, J. Arthur Thompson, John Toll, Merle Antony Tuve, John Hasbrouck Van Vleck, Eugene Paul Wigner, E. J. Williams, Hideki Yukawa; Carnegie Institution of Washington. Dept. of Terrestrial Magnetism, and Institute for Theoretical Physics (Copenhagen).