In this chapter, the basic concepts of both physical-layer security (PLS) and quantum key distribution (QKD) are introduced. The chapter starts with the role of PLS, followed by a brief overview of conventional key-based cryptographic systems. The concept of information-theoretic security is introduced next, and the perfect secrecy condition is described. Computational security is described as a special case of information-theoretic security in which several relaxations are introduced. The concepts of strong and weak secrecy are then introduced. Further, the degraded wiretap channel model, introduced by Wyner, is described, and corresponding wiretap channel codes are defined. Then, the broadcast channel with confidential messages, introduced by Csiszár and Körner, is described, together with the corresponding stochastic code. The last topic in the PLS section is devoted to the secret-key agreement protocol. The QKD section first describes how to break the RSA protocol with the help of Shor’s factorization algorithm, followed by a brief description of foundations for both discrete variable (DV) and continuous variable (CV) QKD schemes. The key limitations of DV-QKD schemes are identified. Various QKD protocols are placed into three generic categories: device-dependent QKD, source device-independent QKD, and measurement device-independent (MDI) QKD. Further, the definition of the secrecy fraction for QKD protocols is provided, followed by a brief description of individual (incoherent) and collective attacks, and explanation of how to calculate the corresponding secrecy fractions. In the section on the organization of the book, a detailed description of the content of the chapters is provided.