Childhood in Russia, family and early schooling; Paris University, first publication, work on crystal growth in Jean Perrin's lab, doctoral thesis. History of Frďřic Joliot-Curie's work, his lab, character, and collaboration with Irene Curie and other scientists in context of pre-war scientific establishment in France; Kowarski's work as Joliot's secretary at the Institut Curie, reactions to Joliot's Nobel Prize; work on magic numbers. Work on fission: Hans von Halban, Enrico Fermi, Otto Hahn, Leo Szilard, ca. 1939; effects of science in wartime France. Applications of fission chain reaction and patents; flight to England with Halban and heavy water supply. Kowarski's integration into English scientific community: James Chadwick, John Cockcroft, the Maud Committee, Marcus Oliphant; course of development of Halban's group in Canada; Kowarski's work between Great Britain and U.S. Return to Europe in 1946; political climate of postwar France, particularly the influence of communism. French Commission on Atomic Energy (CEA), its internal politics, science and scientists in postwar France: Joliot, Pierre Auger, Jean Perrin, Curie, Jules Guřon, Bertrand Goldschmidt. Growth of energy. Family life and marriages; visit to U.S., 1946; comparison of postwar science in U.S. and France. History of French reactors. Kowarski's impressions of French scientific unity and his role in it; introduction of computer technology into nuclear topics; European Nuclear Energy Agency (ENEA); Kowarski's involvement. Contemporaries: J. Robert Oppenheimer, Niels Bohr; developments in CERN and ENEA; reflections on attainment of goals and shortcomings of CERN; thoughts about being a scientist in an international community.