Cognitive Radios (CR) are a concept of radios endowed with an intelligence that allows them to perceive changes in their environment and adapt their parameters to maintain a quality of Service (QoS). While the parameters concerned by the adaptation enclose theoretically all of those of a radio (modulation, coding, …etc.), it is certainly the change of the operating frequency that is the most characteristic of CR. In light of this new requirement, the present paper analyzes the features that CR antennas can have. Then, examples of frequency reconfigurable antennas developed by our group for CR will be presented.