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. This book is useful to power system engineers and graduate students interested in control systems design and modeling of WECSs.