Modern wind turbines generally operate at variable speed in order to maximise the conversion efficiency below rated power and to reduce loading on the drive-train. In addition, pitch control of the blades is usually employed to limit the energy captured during operation above rated wind speed. The higher complexity of variable-speed variable-pitch turbines is offset by the benefits of control flexibility, namely, higher conversion efficiency, better power quality, longer useful life; because of the immediate impact of control on the cost of wind energy, reliable high-performance controllers are essential in making wind technology competitive.