This book describes the electromagnetic theory for the propagating modes of dielectric guides with the objective of understanding the applications of these guides to a telecommunication system. The dispersionless solitonic solution is introduced to inform the reader on this new optical pulse shape which may soon ensure transoceanic communications. The study of the minimization of the waveguide dispersion leads us, by means of several calculated frames, to the weakly-guiding condition. This essential condition for a large bandwidth fibre leads us to the introduction of the practical LP modes. In order to initiate the reader into integrated optics components, the electromagnetic solution for two coupled planar waveguides is treated in an appendix. Another appendix allows the reader to go through a quick initiation of the geometrical optics theory (essential for the study of graded-index fibre), being the iconal equation and the ray equation starting from Maxwell equation under the short wavelength approximation.