The quantum emerged at about the same time as Langevin entered the physics community, in 1900. He followed the development of this new theory from the very outset, attending Marcel Brillouin’sBrillouin, Marcel course at the Collège de France on “Thermodynamics and Radiation” and through the first Solvay Council in 1911 that really brought quantum theory to the core of physics. In 1912–1913 he devoted his annual course at the Collège de France to “The difficulties of the theory of radiation” and he felt deeply upset by the crisis of determinism at the 1927 Solvay Council. In the 1930s he forged a personal interpretation, arguing that it was a crisis of mechanics rather than a crisis of determinismdeterminism.