primary user emulation attack and objective function attack There are also several other types of attacks and the corresponding countermeasures on the physical layer in cognitive radio networks. In this article we first systematically survey different types of attacks on the physical layer and their corresponding countermeasures in cognitive radio networks. Specifically, we summarize six major types of attacks: primary user emulation; objective function attack; learning attack; spectrum sensing data falsification; jamming attack; and eavesdropping.To emphasize the importance of physical layer security issues in cognitive radio networks, we present an analysis method of secrecy capacity in a cognitive radio network model, to characterize how much information a user can transmit safely with the existence of eavesdroppers in cognitive radio networks.