Electromagnetic shielding is frequently used to reduce the emission or to improve the immunity of the electronic equipments against interferences. The ability of an enclosure to perform this task is characterized by its shielding effectiveness (SE). Among all parameters that could be adopted by a designer, shielding is important due to its controllability. The SE of a metal enclosure is determined mainly by its general shape and its apertures. Therefore, this book provides a complete study of the effects of these parameters on SE. Numerical method in electromagnetic is used to find the effect of these parameters on shielding performance. Some practical techniques to enhance the shielding performance are introduced. At the last chapter of the book, a novel method based on matching load has been explained. In this method an aperture load is used to eliminate the resonance frequency of the metallic enclosure.