A number of papers from Victor F. Weisskopf have been collected in this book. The papers included in the first part are dealing with basic concepts in quantum mechanics. Particle-wave duality, quantum scale, and the Niels Bohr works. Papers in the second part describe the recent developments in the physics field during the 20th century: the electron theory, compound nucleus, nuclear structure, and quantum theory of elementary particles. The third part is concerned with peculiar cases: nuclear models, the Lorentz relativistic contraction, light-matter interaction, parity decay, and symmetry. In the fourth part are gathered papers on sciences in general, for which they present a sort of natural philosophy.