This chapter is devoted to the physical-layer security. The chapter starts with a discussion of security issues, followed by an introduction to information-theoretic security, and comparison against the computational security. In the same section, various information-theoretic security measures are introduced, including strong secrecy and weak secrecy conditions. Then, Wyner’s wiretap channel model, also known as the degraded wiretap channel model, is introduced. In the same section, the concept of secrecy capacity is introduced as well as the nested wiretap coding. Further, the broadcast channel with confidential messages is introduced, and the secrecy capacity definition is generalized. The focus is then moved to the secret-key generation (agreement), the source and channel-type models are introduced, and corresponding secret-key generation protocols are described. The next section is devoted to the coding for the physical-layer security systems, including both coding for weak and strong secrecy systems. Regarding the coding for weak secrecy systems, special attention is devoted to two-edge-type LDPC coding, punctured LDPC coding, and polar codes. Regarding the coding for strong secrecy systems, the focus is on coset coding with dual LDPC codes and hash functions/extractor-based coding. The attention is then moved to information reconciliation and privacy amplification. In the wireless channels physical-layer security (PLS) section, the following topics are covered: MIMO fundamentals, wireless MIMO PLS, and secret-key generation in wireless networks. In the section on optical channels PLS, both PLS for spatial division multiplexing (SDM)-fibers-based systems and free-space optical (FSO) systems are discussed.