If Einstein himself could not fully cope with his concept of light quanta, it is no wonder that his contemporaries would not enthusiastically rally behind it from the start either. The early reception of the concept of light quanta starts with initial skepticism by almost the entire scientific community (including many of Einstein’s strongest supporters, e.g., Max Planck and his pupil Max von Laue). Sect. 5.2 interprets the Compton effect 1922/23 as a watershed and Sect. 5.3 discusses the theory by Niels Bohr, H.A. Kramers and John Slater (1924). This BKS theory was the last attempt at a quantum theory without energetically quantized photons—the correlation experiments by Walther Bothe and Hans Geiger (1924–25) refuted it.