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Wireless sensor networks (WSNs) currently operate in the unlicensed industrial, scientific and medical (ISM) spectrum band. However, more and more wireless equipments make this band overcrowded. The tremendous growth in the applications of WSNs has posed great challenges for the precious radio electromagnetic spectrum resource. The limited wireless spectrum resource will be the bottleneck for the large-scale popularization of WSNs. By contrast, the utilization rate of licensed spectrum is very low, and some bands are actually never occupied. It is promising and also challenging for WSNs to adopt the cognitive radio (CR) technology to sense spectrum hole and utilize the vacant frequencies to improve the spectrum utilization rate and communication validity. On the basis of detailed analysis of the involved key technologies for CR-based WSNs, the related solutions are introduced. In addition, a new hardware structure of sensor node is proposed and energy-saving design strategies are discussed.