This textbook attempts to bridge the gap that exists between the two levels on which relativistic symmetry is usually presented – the level of introductory courses on mechanics and electrodynamics and the level of application in high-energy physics and quantum field theory: in both cases, too many other topics are more important and hardly leave time for a deepening of the idea of relativistic symmetry. So after explaining the postulates that lead to the Lorentz transformation and after going through the main points special relativity has to make in classical mechanics and electrodynamics, the authors gradually lead the reader up to a more abstract point of view on relativistic symmetry – always illustrating it by physical examples – until finally motivating and developing Wigner’s classification of the unitary irreducible representations of the inhomogeneous Lorentz group. Numerous historical and mathematical asides contribute to conceptual clarification.