This book describes the development of wavelet-based filter to recognise and identify various types of harmonics in electric power system which creates disturbances in power quality. A hybrid approach involving the wavelet transform, traditional signal processing and rule-based techniques, and other commonly accepted traditional techniques in the power community are designed to construct the filter. The wavelet transform, in particular, plays a significant role in determining the success of the classification task. The wavelet transform is utilised to detect the presence of harmonics in a given waveform. The presence of harmonics is indicated by large magnitudes of discrete wavelet transform coefficients at various scales. These coefficients carry unique disturbance features. These features, along with features provided by traditional techniques, are then utilised by a rule-based approach to determine the disturbance category. The wavelet transform is a powerful tool for detection and mitigation of harmonics in power system.