The book focuses on a recently emerging topic in security. Specifically, the book focuses on physical layer security. Whereas conventional security mechanisms involve cryptography that operates at higher layers of the protocol stack, the methods described in this book secure communications at the physical layer. The area of physical layer security is an emerging field of research that has seen an explosion of research in the past few years. This trend is only expected to continue as there is a growing need to secure wireless communications without conventional cryptographic network security protocols. The proposed book will consist of contributions from roughly a dozen of the leading research groups in this emerging area. For the first time, important results will be collected together. Important topics will include the secrecy rate of MIMO wireless systems, practical codes for the dissemination of secrets, and authentication of wireless transmitters using waveform-level properties.