Constants, such as the gravitational constant and the speed of light, are present in all the laws of physics. Recent observations have cast doubt on one of them. Does this mean that the structure of physics will crumble? Are we seeing the dawn of a scientific revolution? This book is written in the form of an enquiry into the importance of a possible variation in fundamental constants. Jean-Philippe Uzan and Bénédicte Leclercq ask such questions as:What is a constant? What role do constants play in the laws of physics? How can we verify that they are indeed constants?The authors take us though the history of the ideas of physics, evoking major discoveries from Galileo and Newton to Planck and Einstein and raising questions provoked by ever more current accurate observations. They approach physics by way of its constants in order to distinguish the fundamental from the particular, and to recognise different physical forces, but these cannot be drawn together into one unique force, as those seeking a unified theory would like. The book shows how the development of theories leads to simplification, analogy and the regrouping of phenomena. It describes how physicists seek to explain why the world is as it is and why can they cannot explain the values of the mass of elementary particles such as the electron and the proton. The authors ask if we can have confidence in the promising theory of superstrings, which would reinterpret these particles as states of vibration of the strings, extended objects appearing only in macroscopic dimensions. This highly instructive survey of physics, from the laboratory to the depths of space, explores the paths of gravitation, general relativity and new theories such as that of superstrings. It is complete and coherent, and goes beyond the subject of constants to explain and discuss many ideas in physics, encountering along the way, for example, such exciting details as the discovery of a natural nuclear reactor at Oklo in Gabon.