This chapter presents EMI filters whose task is to block EMI currents to leave electric and electronic equipment and EMI-contaminated neighboring equipment. Additionally, the task of an EMI filter is to protect the equipment itself from EMI currents produced by other equipment and/or electromagnetic events, such as electric power transients, ESD, and lightning. Fundamental EMI filter parameters, such as the frequency response (or operating bands) and insertion loss, are described herein as well. With respect to operating bands, four types of filters are considered here: low-pass, bandpass, high-pass, and bandpass filters. EMI filters are designed with different configurations and different numbers of electric passive, reactive and resistive components. The number of filter components can range from a single, for lower insertion loss, to multiple components, for higher insertion loss. In this chapter, various configurations of EMI filters are considered. Additionally, EMI filters, which are used for simultaneously suppressing differential-mode (DM) and common-mode (CM) currents, are considered in this chapter. These DM/CM filters are used mainly for power mains applications and are known as power mains filters.