Board level shielding (BLS) is widely used in various electronic products or systems. They play an important role in solving EMC problems (e.g., EMI radiation, intra-system interference, RF problems). With the upgrading of end user requirements, the design requirements for BLS are becoming increasingly stringent. Customers now demand a higher frequency band, lighter weight, smaller size or low-height shields, and more. Therefore, how engineers evaluate the shielding effectiveness (SE) of BLS products has become crucial. This paper summarizes key factors that affect board level shielding performance through actual simulations and tests. It should provide a useful reference for engineers to design or choose EMI shielding solutions.