Why could it be useful to think about the complex history of a concept like the photon? The dense stratification of a dozen layers of meaning, which are fused together in this concept is still a live issue. For a deeper understanding it is instructive to study the history behind the concept and the cognitive obstacles faced by some of the world’s most brilliant physicists. Participation in those profound, sometimes heated debates opens deep insights into the way in which our conceptual apparatus operates, into the genesis of new concepts and new mental models. This introduction explains the method employed by this study, particularly the general historiography of scientific terminology and the treatment of mental models from cognitive psychology. Sect. 1.2 draws clear defining lines between the labels ‘term’, ‘concept’ and ‘mental model’. Sect. 1.3 interprets the way a concept develops as layered semantic accretion. Specific preliminary studies by the author and other historians of science on individual historical episodes are described with full citations. Chap. 1 closes with the author’s acknowledgements to publishers, institutions and individuals.