In this chapter, we investigate several CEM techniques for modeling electromagnetic fields inside cavities using EMA3D. We start with a simplest canonical model, and move on to a somewhat more sophisticated model of a Boeing 707. The effects of absorption losses and aperture losses on the shielding effectiveness are studied. In addition, statistical studies of electric fields inside the canonical models are conducted at higher frequencies (up to 12 GHz), the conditions for considering the fields as statistical are also presented, and the power balance method is introduced for frequencies too high to be simulated using FDTD.