With the rapid growth of wireless communication services, the radio spectrum is becoming increasingly overcrowded. Cognitive radio is regarded as a promising approach to alleviate the spectrum shortage and improve the spectrum utilization. Thus, the cognitive radio recently becomes one of hot research sports in wireless communications area. Based on the extensive investigation, the research progresses made in cognitive radio during the past ten years are reviewed from the aspects of spectrum policy evolution, network structure and standard development, and the related work. The relevant literatures and the famous international research institutes that focus on the field of cognitive radio are listed. Then, the application prospects of cognitive radio are presented. It is shown that cognitive radio could provide spectrum resource for the emerging applications, such as smart grid networks, wideband cellular networks, public safety networks, and medical body area networks. The cognitive radio technology is able to promote the development of these new emerging communication services. In summary, cognitive radio has wide application prospects and is expected to be one of the 5G communication standards. Furthermore, the future research directions in the area of cognitive radio are put forward, which include robust spectrum sensing and cognitive relay networks based on multiple user cooperation. Therefore, a good guidance for the researchers in the field of cognitive radio is provided.