An antenna differs fundamentally from the other devices in this book, because the currents flowing on its surface are meant to radiate and interact with other antennas. The source of these currents is often a transmission line, and thus, an antenna can be viewed as a load excited by current that flows across the antenna s conductors and returns to ground. Commonly, an antenna is designed assuming it exists in a region of free space, with no other conductive objects present. Yet, every antenna is attached to another conducting object such as the metal housing containing its transmitter or receiver, and most antennas sit near the surface of the Earth, which has significant conductivity. As currents flow on antennas, they are likely to interact with the currents that flow on such other conductors. Most of these conductors, whether connected or disconnected from the antenna behave as ground planes, and it is their influence on antenna performance that is the subject of this chapter.