Electromagnetic shielding of enclosures is simulated in this PhD thesis. Metallic enclosures with a frontal aperture have been implemented and shielding effectiveness has been calculated in frequency and time domains. The CST Microwave Studio application has been used, and necessary electromagnetic shielding measurements have been implemented in order to confirm the simulated results. An anechoic chamber and the network vector analyser ZVA 67 R&S have been employed. There were different setups that consist on two shielding enclosures with different apertures on their frontal walls, as well as an electric and a magnetic probes, and an external log-periodic antenna. ABSTRACT Finally, in order to get more generic solutions that can be useful to later SE studies, the new SE results have been analysed and interpreted for an aperture size scanning that provide an unique value for the more critical SE indicator and for an specific bandwidth allowing direct SE comparisons with other enclosures. AGRADECIMIENTOS Es un honor para mí expresar mi agradecimiento a Antonio Lozano por su tiempo y atención dedicados a la dirección de las tareas investigadoras de este trabajo. Es necesario para mí mostrar un profundo agradecimiento a mis hijos Ezequiel y Lorena y a mi madre porque, a pesar de los ratos y detalles que nos han faltado, he sentido durante todo este periodo su amor incondicional. Por último, soy consciente de la bendición de contar con un manojo de amigos y amigas que confían en mí y me animan en esta y en otras batallas de esta gran escuela que es la vida. A todos ellos, muchas gracias y un fuerte abrazo.