The proliferation of Non-Linear Loads is the main reason for harmonic currents injection and flow in the electric power system. This condition causes a host of detrimental effects including increased power loss, electromagnetic interference, resonance, and relay malfunctioning. As a result, the quality of the energy delivered to the consumers is degraded. Static and Active Filters are used as mitigating tools: these devices sink the harmonic currents injected by the Non-Linear Loads, thus resulting in a grid current at fundamental frequency only. This book deals with the use of Wind Energy Conversion Systems (WECSs) as Active Filters: this application is promising since the number of installed Non-Linear Loads and of WECSs is continuously increasing and the latter operates at full power only for a limited time of the year. The effects of the proposed application on WECSs' performance in both steady-state and transient operation are addressed and design issues are identified. Thisbook is useful to power system engineers and graduate students interested in control systems design and modeling of WECSs.