This chapter presents a general concept of EMI shielding, which is divided into near-field shielding and far-field shielding, depending on whether the shield is in the near-field or far-field region of an EMI source. Additionally, this chapter defines the shielding effectiveness (SE), which is a sum of three losses: reflection loss (R), absorption loss (A), and multireflection loss (MR). These losses are considered separately and numerically illustrated in this chapter. Additionally, improvement of the SE by double-layer shielding is considered. Furthermore, the impact of apertures in equipment enclosures (such as ventilation openings and slots on chassis) on the shielding effectiveness is considered and numerically illustrated, including the SE improvement of ventilation panels by using waveguides as EMI attenuators.