Originally published in 1960, and written by the renowned German-American physicist Alfred Landé (1888–1976), this book challenges the view that the principles of wave-particle duality, complementarity and uncertainty are fundamental to micro-mechanics. Instead, the text sets out to reduce dualistic quantum phenomena to still more elementary general principles, such as symmetry, invariance and continuity. It thus develops the Bohr-Heisenberg account, resting on Born's statistical interpretation, into a unitary theory. Appendices are included. This book will be of value to anyone with an interest in quantum physics and the history of science.