Arnold Sommerfeld (1868-1951) is considered one of the founders of modern theoretical physics and trains generations of important physicists. He studies in Königsberg, where he gains his doctorate in mathematics in 1891. He is influenced above all by his subsequent time as assistant to Felix Klein in Göttingen. In 1900 he is appointed professor of technical mechanics at the Technical University of Aachen, and full professor of theoretical physics at the University of Munich in 1906. He starts incorporating Einstein’s special theory of relativity into his lectures as early as 1907, and writes important articles on refining its mathematical formalism. He is among the early supporters of the quantum hypothesis and further develops the Bohr model of the atom. Sommerfeld’s fundamental works include his 1919 treatise Atomic Structure and Spectral Lines, that is regarded as the “bible” of atomic physicists at that time.