Power is very important to modern systems. From the smallest sensors, bionic implants, laptops, consumer products to satellites and oil platforms, it is important to be able to deliver power means other than classical wires or transmission lines. Wireless transmission is useful in cases where instantaneous or continuous energy transfer is needed, but interconnecting wires are inconvenient, hazardous or impossible. An efficient method for wireless power transfer would also enable advances in such diverse areas as embedded computing, mobile computing, sensor networks and micro robotics.