The aim of this work is to study the principles upon which the classical and relativistic theories of the electromagnetic and gravitational fields are based. Thus, the primary object of the book is to present a simple exposition of Maxwell's theory, of General Relativity and of the link between those two concepts, namely, Special Relativity. In the nineteenth century the notion of a continuous field gradually replaced the idea of action at a distance. The electromagnetic theory that was elaborated at that time covers a very large area of Physics, since it makes possible the description of permanent phenomena, electrostatics and magnetostatics, as well as of variable phenomena. It anticipates the existence of waves, and thereby the theory of light is annexed to this vast domain. It was discovered that Maxwell's equations changed their form when they were related to reference systems associated with two observers in rectilinear uniform motion with respect to each other and each endowed with the absolute time required by classical mechanics. This was a most remarkable fact. Indeed, as soon as attempts were made to verify the results of classical kinematics by means of experiments with the propa- gation of light, there arose a whole series of contradictions.