Due to the broadcast nature of wireless channels, security and privacy is of utmost concern for future wireless technologies. However, how to securely transfer confidential information over a wireless network in the presence of eavesdroppers that may intercept the information exchange between legitimate terminals still remains a challenging task. Although security was originally viewed as a high-layer problem to be solved using cryptographic methods, physical (PHY) layer security is now emerging as a promising means of defense to achieve wireless secrecy in communications. In PHY layer security, the core idea is to exploit the characteristics of wireless channels such as fading or noise to design efficient secure transmission strategies, such that the message from the source to the intended receiver is kept confidential from both passive and active eavesdroppers. Over the past few years, PHY layer security has been widely recognized as a key enabling technique for secure wireless communications in future networks.