2015 Reprint of 1930 Edition. Full Facsimile of the original edition. Not reproduced with Optical Recognition Software. In this classic account, based on lectures delivered at the University of Chicago, Heisenberg presents a complete physical picture of quantum theory. Quantum theory is a fundamental branch of physics which describes physical phenomena at scales typical of the quantum realm of atomic and subatomic length scales, where the action is on the order of the Planck constant. At these scales, many everyday concepts concerning physical objects and energy (including the photons making up visible light) are believed to behave and interact extremely differently than is usually seen in daily life.