This book deals concisely with all the theories of contemporary physics, from relativity (restricted and general) to quantum mechanics and quantum field theory, via nuclear and particle physics. In addition, the rudiments of plasma physics, solid state physics, semiconductor physics, and group theory are exposed. The approach of this textbook is to deal directly with the mathematical formalism and to present the main equations of each theory, explaining their meaning and consequences as well as the problems left open. The logical thread linking the different paragraphs is given by an overcoming of the old theories to meet the new requirements; meanwhile, two main directives in contemporary physics will be emphasized: the trend toward the unification of physical theories and the fundamental role of energy in the equations underlying each of them.