Antennas are essential and crucial components of radar and wireless communication systems, but sometimes their inability to adapt to new operating conditions can hamper system performance. These limitations can be mitigated or eliminated, and reconfigurable antennas can bring new levels of functionality to any system by enabling it to modify its behavior in response to changing system requirements or environmental conditions. Depending on the physical property that makes the antenna reconfigurable, the antennist needs to choose very precise reconfiguration techniques and also the necessary polarization circuits. In accordance with the reconfigurability functions they offer, reconfigurable antennas can be classified into one of four categories: frequency reconfiguration, radiation pattern reconfiguration, polarization reconfiguration and hybrid reconfiguration. This classification will be presented in this chapter, preceded by the principle and main reconfiguration techniques encountered in the literature, as well as the polarization circuits of the active elements required.