Five chapters of this dissertation concentrate on the characterization and cancellation of high frequency parasitic parameters in EMI filters. One chapter addresses the interaction between the power interconnects and the parasitic parameters in EMI filters. The last chapter addresses the characterization, evaluation and design of noise separators. Both theoretical and experimental analyses are applied to each topic. This dissertation tries to explore several important issues related to EMI filters and noise separators. The author wishes to find some helpful approaches to benefit the understanding and design of EMI filters. The contributions of the dissertation can be summarized below: 1) Identification of mutual couplings and their effects on EMI filter performance 2) Extraction of mutual couplings using scattering parameters 3) Cancellation of mutual couplings to improve EMI filter performance 4) Cancellation of equivalent series inductance to improve capacitor performance 5) Analysis of mode transformations due to the imperfectly balanced parameters in EMI filters 6) Analysis of interaction between power interconnects and EMI filters on filter high-frequency performance 7) Modeling and design of high-performance noise separator for EMI diagnosis 8) Identification of the effects of parasitics in boost PFC inductor on DM noise Although all topics are supported by both theory and experiments, there may still be some mistakes in the dissertation. The author welcomes any advice and comments. Please send them via email to shuowang@ieee.org. Thanks / Ph. D.