This book provides a panoramic view from 1927–1938 of the development of a physical theory that has been on the cutting-edge of theoretical physics ever since P. A. M. Dirac’s quantization of the electromagnetic field in 1927: quantum electrodynamics. Like the classic papers chosen for this volume, the introductory Frame-Setting Essay emphasizes conceptual transformations which carried physicists to the threshold of renormalization theory. The published papers and correspondence of Bohr, Heisenberg, Dirac and Pauli provide a fascinating analysis of the meaning and structure of a scientific theory. This book goes beyond the historical and philosophical into current physics. Unavailability of English-language versions of certain key papers, some of which are provided in this book, has prevented their implications from being fully realized. Awareness of research from sixty years ago could well provide insights for future developments.