This volume contains some of Professor Wigner's more popular papers, which, in their diversity of subject and clarity of style, reflect the author's deep analytical powers and the remarkable scope of his interests. Included are articles on the nature of physical symmetry, invariance and conservation principles, the structure of solid bodies and of the compound nucleus, the theory of nuclear fission, the effects of radiation on solids, and epistemological problems of quantum mechanics. Other articles deal with the story of the first man-made nuclear chain reaction, the long-term prospects of nuclear energy, the problems of Big Science, and the role of mathematics in the natural sciences. In addition, the book contains statements of Wigner's convictions and beliefs, as well as memoirs of his friends, Enrico Fermi and John von Neumann.