Wind at higher altitude is stronger and steadier compared to wind accessible by conventional wind turbines. It has been recognized as a major source of renewable energy with the potential to cover the complete global energy demand. Computer-controlled tethered airfoils (kites) driving generators on ground are a promising technology to exploit this energy. In contrast to conventional wind turbines, kite power systems can be lightweight, inexpensive, mobile and flexible in use. They have a significantly higher capacity factor and a lower environmental footprint. Possible applications range from small 10 to 20 kW systems operating below 500 m altitude, up to large-scale high-altitude wind farms generating Megawatts of electricity from wind at 8 km altitude.