This two-volume book is on the genesis of quantum mechanics. The first volume covers the key developments in the period 1900-1923, which provided the scaffold on which the arch of modern quantum mechanics was built. This volume traces the early contributions by Planck, Einstein, and Bohr to the theories of black-body radiation, specific heats, and spectroscopy, all showing the need for drastic changes to the physics of their day. It examines the efforts by Sommerfeld and others to provide a new theory, now known as the old quantum theory. After some striking initial successes (explaining the fine structure of hydrogen, X-ray spectra, and the Stark effect), the old quantum theory ran into serious difficulties (failing to provide consistent models for helium and the Zeeman effect) and eventually gave way to matrix and wave mechanics. The second volume provides detailed analysis of the classic papers by Heisenberg, Born, Jordan, Dirac, De Broglie, Einstein, Schrödinger, von Neumann and other authors. Drawing on the correspondence of these and other physicists, their later reminiscences and the extensive secondary literature on the "quantum revolution," this volume places these papers in the context of the discussions out of which modern quantum mechanics emerged. It argues that the genesis of modern quantum mechanics can be seen as the construction of an arch on a scaffold provided by the old quantum theory, discarded once the arch could support itself.