Correspondence; memoranda; scientific notes and notebooks; diaries; drafts and texts of speeches; lecture notes; publications. The collection documents Kowarski's career in nuclear physics during and after World War II. It covers his work in the laboratory of Frďřic Joliot in Paris, especially research on the uranium fission chain reaction (1939-1940), and subsequent patent investigations (with copies of papers concerning the work of Enrico Fermi and Leo Szilard); his war work on nuclear reactor development in Cambridge, England with Hans Halban, and at Chalk River, Canada; and his postwar work as a leader of the Commissariat a' l nergie Atomique (1946-1952), a founder and administrator of laboratory services and data processing at the European Organization for Nuclear Research (CERN); and a participant in international high-energy affairs, including work with the European Nuclear Energy Agency (ENEA). Also reflects his active involvement in the controversy over civilian nuclear energy. The addition to the collection covers primarily his post 1950 work and chronicles the developments in the international community of atomic scientists, including the movement of European scientists; the application of reactor technology; the social impact of nuclear technology; the development of data processing devices in high-energy physics, international cooperation; and peaceful uses of atomic energy. Correspondents include: Luis Alvarez, Otto Robert Frisch, Samuel Goudsmit, William Frederick Miller, Joseph Rotblat, Maurice Shapiro, Victor Weisskopf, Eugene Wigner.