The subtitle of this volume is exact, for Leo Szilard's work ranged from physics to biology; he crossed over with ease from the world of neutrons to that of neurons, from the nuclei of atoms to the nuclei of cells. At a deeper level, all of these research activities converge, unified as they are by Szilard's interest in the fundamental ideas of order and organization that underlie them all. It is no accident, as Jacques Monod notes in this foreword, that this interest should appear explicitly as the subject of one of his very first papers, and of his last. In the former, concerning the operations of an intelligent being on a thermodynamic system, Szilard anticipated by many years the entropic formulation of information theory, while the latter, "On Memory and Recall," is concerned with the organization of the central nervous system. Szilard's spontaneous, unrepressed flow of ideas was followed by a critical process of selection and refinement, which may account for the relatively small number of published papers—under thirty—found in this collection. The papers are arranged chronologically, and their ordering comprises a resume of Szilard's scientific life. The first group of papers, 1925-1939, all deal with various aspects of physics—thermodynamics, x-ray crystallography, and nuclear physics. The work in this last area includes studies of neutron bombardment and of the possibilities of nuclear fission and chain reactions, which Szilard was among the first to envision. The second group, 1940-1945, comprises the first publication of documents relating to the Manhattan Project, and reflect Szilard's part in it. Most of these documents were previously classified. There are no scientific papers from the years 1946-1948. These were years when Szilard was intensely active in the political and social arena, attempting to bring the atomic bomb under international control. Documents relating to this struggle, and to his earlier leadership in efforts both to undertake the atomic project and to prevent the bomb's use without warning against Japan, will be collected in another volume. It was also during these critical years that Szilard's interest decisively turned from physics to biology. The third group covers the years 1949-1964, when Szilard's interest centered around biological problems. Among these are the regulation of cellular metabolism, antibody formation, the aging process, and the functioning of the central nervous system. A final section of the book reproduces patents and patent applications of historic importance dating from 1923 to 1959, and ranging from the invention of a refrigerator, jointly with Albert Einstein; an application anticipating the cyclotron in 1928; and the general concepts of a nuclear chain reaction in 1934 to the first patent on the nuclear reactor granted jointly to Fermi and Szilard.