The presence of harmonics in power lines results in greater power losses in the distribution system, interference problems in communication systems and, sometimes, in operation failures of electronic equipments. Active filters are special equipments that use power electronic converters to compensate for current and/or voltage harmonics originated by non-linear loads, or to avoid that harmonic voltages might be applied to sensitive loads. In this book, Synchronous Reference Frame (SRF) theory is implemented for the generation of reference current signals for the controller. This book also illustrates how the functional definition of a switching converter can be used to model active filters. This book presents the comparative performance evaluation of the fixed hysteresis band current controller and the adaptive hysteresis band current controller for a three phase three wire shunt active power filter and inspects the effectiveness of the filter in harmonics currents suppression for both cases.