For this relatively short talk, his second at the Lindau Nobel Laureate Meetings, Alfred Kastler chose to give a detailed historical overview of a development in physics of fundamental interest to his young audience. This development took place in Berlin during a few weeks at the end of year 1900 and the main actor was the theoretical physicist Max Planck, at the time a little bit more than 40 years old. Planck was an expert on classical thermodynamics, as formulated by Rudolf Clausius with entropy as the basic entity some years earlier. At Planck’s age, few theoretical physicists step outside of their expertise. But during the few hectic weeks described by Kastler, Planck reluctantly opened up for an alternative statistical formulation pioneered by Ludwig Boltzmann. The outcome was, in many ways, the starting point for the theory of quantum physics, the most important physical theory to be formulated in the 20th century. Kastler goes through the detailed mathematical derivations, using slides and a blackboard, which makes it quite difficult to follow all the details. But the main message is clear and the importance of the resulting formula describing the spectral distribution of black-body radiation is discussed. It is interesting to note that the formula contains two atomic parameters that Planck could calculate by comparing with the best experimental measurements on laboratory black-body radiation available year 1900. This almost gave him the Nobel Prize in Physics of 1908, but the Royal Swedish Academy of Sciences instead decided to wait and instead awarded him the physics prize of 1918 for his discovery of energy quanta. The same formula was, in a sense, again tested with extreme accuracy on the black-body radiation of the Universe by the satellite COBE in the beginning of the 1990’s, leading to a Nobel Prize in Physics for John Mather in 2006!