This chapter describes harmonic producing loads, effects of harmonics, and harmonic mitigation methods, especially using active filters. It discusses different topologies of active filters, their applications, configurations, control methods, modeling and analysis, and stability issues. Active filters have been known as the best tool for harmonic mitigation as well as reactive power compensation, load balancing, voltage regulation, and voltage flicker compensation. Mitigation or cancellation of harmonics can be done by using passive or active filters. The increased severity of power quality problems and other problems associated with the passive filters such as large size and weight, higher cost, fixed compensation, and resonance problems with loads and networks have required a focus on a power electronic solution, that is, active filters. An active filter can utilize current source inverters (CSI) or voltage source inverters (VSI). CSI-based active filters employ an inductor as the energy storage device. VSI-based active filters use a capacitor as the energy storage device.