The synthesis of quantum mechanics and relativity into a unified theory is one of the greatest achievements of Physics in our century. In fact, only within the framework of this synthesis is it possible to understand fundamental characteristics of our world such as the Pauli principle, the existence of matter and antimatter, as well as the possibility of production and annihilation of particles in nuclear reactions that produce, among other things, the stellar energy to which our very existence is owed. Without the assistance of special relativity, quantum laws alone would be unable to create a Universe capable of self-awareness. This book is an attempt to present this magnificent synthesis at a level that requires nothing more than the basic knowledge of an undergraduate course in quantum mechanics. Without the usual use of an intimidating formalism, the book attempts to highlight the simplicity of the basic ideas and create a "friendly atmosphere" that will allow the average student to overcome their fears about the difficulty of the subject and attempt the ascent to that privileged peak that offers a comprehensive view of our exquisitely quantum and relativistic universe.