According to the approach described in Chap. 2, studying the development of a concept entails the study of scientific practices as well as epistemic and social constellations. For the reader not familiar with the development of the community of theoretical physicists in the years from 1923 to 1949, this chapter will give an outline of this network. It will also serve as a backdrop when the overall development fades into the background in the in-depth discussion in the chapters to come. I will describe the development of the community of practitioners starting shortly before the development of quantum mechanics and reaching out to the shift of research centres to the US in the post-WWII period.