The fundamental laws of physics include fixed numerical parameters called "universal constants". These quantities, determined experimentally and supposed to vary neither in time nor in space, remain one of the greatest mysteries of science. Our current knowledge does not allow us to understand their origin nor to determine them in a theoretical way. In this book, the author is mainly interested in these elements. His approach is to build a theoretical framework that could explain the source of these constants in our laws of physics and to propose a method that would not only predict them all but also define them. To do this, the author begins by introducing the assumptions on which he bases his theory. Then, in a first part, he expresses the universal constants, the Planck units and the thermodynamic quantities of the Universe in terms of its characteristic variables, such as its mass and its radius. In a second part, he demonstrates the constancy of the Planck units as well as that of the universal constants and, finally, he proposes a general theoretical framework to determine them.