We now come to the most successful physical theory of all time. Fifty years have elapsed since the ‘new’ quantum theory emerged from the ‘old’ quantum theory of Niels Bohr. It has been used to explain the properties of semiconductors and transistors and has given rise to the field of quantum electronics; it has been applied to gases, to stars and to atomic nuclei, and it has never failed; put more carefully, the inaccuracies of the results have never exceeded values regarded as reasonable.