Cognitive radio system (CRS) is a radio system which is aware of its operational and geographical environment, established policies, and its internal state. It is able to dynamically and autonomously adapt its operational parameters and protocols and to learn from its previous experience. Based on software-defined radio (SDR), CRS provides additional flexibility and offers improved efficiency to overall spectrum use. CRS is a disruptive technology targeting very high spectral efficiency. This paper presents an overview and challenges of CRS with focus on radio frequency (RF) section. We summarize the status of the related regulation and standardization activities which are very important for the success of any emerging technology. We point out some key research challenges, especially implementation challenges of cognitive radio (CR). A particular focus is on RF front-end, transceiver, and analog-to-digital and digital-to-analog interfaces which are still a key bottleneck in CRS development.