This book introduces the fundamentals of physical layer security (PLS) and demonstrates how a variety of PLS techniques can be applied to improve the security of wireless communication systems. In particular, this book covers three security aspects of wireless communications. It includes secrecy, i.e., preventing eavesdroppers from intercepting information from transmitted wireless signals, covertness, i.e., hiding the transmitted signals themselves from malicious wardens and authentication, i.e., authenticating the identities of communicating entities. When discussing the secrecy of wireless communication systems, this book covers physical layer secure communication in multiple-input multiple-out (MIMO) systems based on beamforming and precoding techniques, in relay systems based on link/relay selection and in large-scale random networks based on cooperative jamming. Regarding the covertness of wireless communication systems, this book introduces physical layer covert communication in relaying systems and MIMO systems. Also, when discussing authentication in wireless communication systems, this book introduces the implementation of physical layer authentication in MIMO systems based on channel features and/or radiometric features of transceivers. In addition, this book presents security-aware routing in wireless networks based on physical layer secure communication techniques. This book targets researchers in the fields of physical layer security and wireless communications security. Advanced-level students in electronic engineering or computer science studying these security topics will also want to purchase this book as a secondary textbook.