The term 'power quality' refers to the purity of the voltage and current waveform, and a power quality disturbance is a deviation from the pure sinusoidal form. Harmonics superimposed on the fundamental are one cause of such deviations. The widespread and increasing use of solid state devices in power systems are leading to escalating ambient harmonic levels in public electricity supply systems.This book provides the details of causes, effects and mitigation methods of harmonics in power systems.Also, gives the idea of modeling of an active power filter using three different harmonic extraction methods with new hysteresis switching strategy. The complete details of MATLAB based simulation studies and discussions are presented.