The analysis of non-linear systems in the frequency domain is studied and a new class of filters, called energy transfer filters, is introduced. While conventional linear filter design procedures are based on the principle of attenuating unwanted effects the new energy transfer filter design concept exploits non-linearity to allow energy to be moved to new frequency locations. The ability to design non-linear filters that can move energy to designed locations in the frequency domain introduces new degrees of freedom into filter design and offers new solution possibilities to many filtering problems.