In one of his lectures on the development of physics Max Planck said: "In the history of science a new concept never springs up in its complete and final form as in the ancient Greek myth, Pallas Athene sprang up from the head of Zeus." The history of physics is not only a sequence of experimental discoveries and observations, followed by their mathematical description; it is also a history of concepts. For an understanding of the phenomena the first condition is the introduction of adequate concepts. Only with the help of correct concepts can we really know what has been observed. When we enter a new field, very often new concepts are needed. As a rule, new concepts come up in a rather unclear and undeveloped form. Later they are modified, sometimes they are almost completely abandoned and are replaced by some better concepts which then, finally, are clear and well-defined. I would like to describe this development in three cases which have been important for my own work. First, the concept of the discrete stationary state, which obviously is a fundamental concept in quantum theory. Then, the concept of state, not necessarily stationary or discrete, which only could be understood after quantum mechanics and wave mechanics had been developed. And finally, closely connected with the first two, the concept of the elementary particle which is under discussion until now.