Yannis Iliopoulos, one of the co-creators of the Standard Model of fundamental particles and forces, explains step-by-step, with clarity and precise emphasis on the essentials, everything required to understand the physical process that, in the first moments of cosmic evolution, imparted non-zero masses to most elementary particles. The unprecedented publicity caused by the discovery of the Higgs particle at CERN in 2012 is precisely because it was the long-awaited tangible evidence of this process, which is crucial for the form of the world. After introducing the necessary knowledge of modern cosmology, Yannis Iliopoulos clarifies the concept of physical symmetries and their breaking during phase transitions, as well as their role in the formation of the Standard Model. He highlights how three basic elements of this formation each lead separately to mass problems, as they require the masses of almost all elementary particles to be zero. He then explains the "miraculous" physical process that solves the mass problems when these three elements are combined into a unified dynamic, as happens in the Standard Model and, more generally, in theories with local symmetries and spontaneous breaking of selected symmetries. Finally, after reviewing the experimental confirmations of the Standard Model, the author explains how we are led today to the convincing connection of early cosmological phenomena with high-energy physics at the microscopic level. Two Appendices summarize the basic information required for a fuller understanding of the book by readers without prior background knowledge. *** Yannis Iliopoulos is Emeritus Research Director of CNRS, a member of the Theoretical Physics Laboratory at the École Normale Supérieure (Paris)