Electromagnetic (EM) shielding has been used extensively in electronic products to reduce the radiated electromagnetic interference (EMI) from high-speed digital circuits so that the products could comply with the international EMI regulatory limits. Good estimation of EM shielding for a wide frequency band is an important area of the product design. Unfortunately, estimation of EM shielding is a complex problem because of many factors, such as distance between the shield and the EMI source, nature of the EMI source, location of the source, cavity resonance, frequency and aperture sizes, etc., could influence the shielding effectiveness of a shield. Due to the complexity, in many cases, it is difficult to estimate EM shielding based on analytical or semi-analytical methods. In this thesis, an electromagnetic code, based on the Method of Moments (MoM) technique, is developed and validated. Its capability to study EMI issues related to enclosure shielding and heat sink has been demonstrated.