With the development of the cognitive radio, that is a system (transceiver) aware of its own operational capabilities and needs which aims at a more efficient use of radio resources (e.g., more universal solution for the co-existence of multiple radio standards, but also limitation of electromagnetic radiations), the antennas with reconfiguration capabilities will become unavoidable. This chapter describes the different types of existing reconfigurable antennas and related approaches allowing the reconfiguration. The presentation relies on many examples from the literature and an example of designing a complete system of reconfigurable antenna frequency is finally detailed.