Electromagnetic Interference (EMI) is an unwanted effect between two electrical systems as a result of either electromagnetic radiation or electromagnetic conduction. EMI is the major adverse effect caused by the application of switch mode power supplies (SMPS). In switching power supplies, EMI noise is unavoidable due to the switching actions of the semiconductor devices and resulting discontinuous currents. EMI control is one of the more difficult challenges in SMPS design, beyond functional issues, robustness, cost, thermal and space constraints. This paper focuses on the theory and mitigation techniques of the conducted portion of EMI, specifically as generated by a step-down switcher.