  In the midst of Weyl’s profound research on Lie groups there occurred a great revolution in physics, namely the development of quantum mechanics. We shall perhaps never know Weyl’s initial reaction to this development, but he soon got into the act and studied the mathematical structure of the new mechanics. There resulted a paper of 1927 and later a book, this book together with Wigner’s articles and Gruppen Theorie und Ihre Anwendung auf die Quanten Mechanik der Atome were instrumental in introducing group theory into the very language of quantum mechanics.