Aim of this thesis is that to furnish some methodologies for the accurate evaluation of the electromagnetic characteristics of a microstrip. This results particularly useful for the extraction of equivalent circuital parameters, needed from circuit simulators to study more complex structures. From an analysis of the available models in literature, it is possible to find that some approximations are usually assumed to simplify the treatment. It can be observed that it is usual to neglect the conductive losses, that nevertheless result more and more remarkable with the increase of the density of the circuits. Besides, the existing models for the analysis of microstrips don't generally take into account the thickness of the structure, if not in a perturbative way. This thesis has the objective to propose some models that try to eliminate or however to reduce the approximation around the finite conductivity and the finite thickness of a microstrip. In the first chapter, general aspects of the microstrips are introduced, shortly examining its uses and the constructive aspects. In the second chapter a lossless microstrip of infinitesimal thickness is examined. Different kinds of sources are examined. The aim of the chapter is to introduce the method that will be used in future. In the third chapter, a microstrip of infinitesimal thickness and finite conductibility is examined. Then the method is generalized to the case of an arbitrary number of coupled microstrips. In the fourth chapter, it is shortly presented a variation of the method previously exposed, studying the problem of a thick strip in the free space. Then, a microstrip of finite thickness is examined. In the thesis, also the computational aspects related to the convergence of the method and to the numerical treatment of the integrals.