The book deals with non-relativistic quantum mechanics. This physical theory arose in 1925-7 from earlier very fragmentary approaches to the quantum theory, and it underwent a remarkable development, in the following respects. Firstly, although it is an adequately complete theory as regards its field of application, as soon as it had been constructed it was thought to be in need of extension, since it did not take account of two important topics: the quantization of classical fields, and relativity. Thus, in addition to its application in atomic physics and especially in nuclear physics, it was soon the subject of a great variety of attempts to enlarge it in these two directions. This extension of non-relativistic quantum mechanics to form a relativistic quantum theory, using the field concept, is of course a purely physical development, which moreover cannot yet be regarded as completed. Secondly, non-relativistic quantum mechanics itself led to quite deep and continually revived debates of a more philosophical type regarding certain epistemological and ontological principles of the older classical physics, since it seemed that these principles were no longer tenable. These debates have a significance to physics which is by no means negligible, despite their philosophical flavour, since they essentially concern directly the question of the content of the new mechanics. Like the purely physical further development of quantum mechanics, they are today, more than forty years after the theory was established, still far from being settled. There is thus the remarkable situation that a vigorous discussion continues among those more inclined to the philosophical type of basic research, regarding the content of a theory that has long been left behind by the main stream of research in theoretical physics.