Foreword to the English edition

The lectures which I gave at the University of Chicago in the spring of 1929 afforded me the opportunity of reviewing the fundamental principles of quantum theory. Professor Heisenberg's Chicago lectures on "The Physical Principles of the Quantum Theory" can now reach a wider audience than could attend them when they were originally delivered. Professor Heisenberg's leading place in the development of the new quantum mechanics is well recognized by those who have been following its growth. It was in fact he who first saw clearly that in the old quantum theory, there have been no essential changes in these principles since the conclusive studies of Bohr in 1927, and many new experiments have confirmed important consequences of the theory, such as the Raman effect. However, even today, the physicist often has a kind of faith in the old.