Quantum key distribution (QKD) is a quantum information task that has reached a level of maturity suitable for commercialization. It enables the creation of a secret key between authorized parties connected by a quantum channel and a classical authenticated channel. The security of the key can be guaranteed in principle without restricting the eavesdropper's capabilities. This review provides an updated, practical overview of QKD, focusing on its theoretical foundations and experimental implementations. The paper begins with an introduction to cryptography and the basics of QKD, including its generic setting, the origin of security, the choice of light, the BB84 protocol, and an example of eavesdropping. It then discusses the elements of practical QKD, including milestones, generic protocols, security notions, and explicit protocols. The review covers discrete-variable, continuous-variable, and distributed-phase-reference protocols, detailing their security proofs, bounds, and practical considerations. The paper emphasizes the importance of theoretical tools in assessing the security of QKD protocols, including the secret key rate, the impact of losses, and the challenges of eavesdropping. It also addresses the practical aspects of QKD, such as the choice of light, sources, physical channels, detectors, and synchronization. The review highlights the differences between discrete-variable and continuous-variable protocols, as well as the challenges of distributed-phase-reference protocols. The paper concludes with a discussion of the future perspectives of QKD, including finite-key analysis, open issues in unconditional security, black-box security proofs, and the potential for longer distances through satellites and repeaters. It also compares different experimental platforms and discusses the importance of practical considerations in the implementation of QKD. The review underscores the need for a balance between theoretical security proofs and practical implementation, highlighting the complexity of comparing different QKD platforms.Quantum key distribution (QKD) is a quantum information task that has reached a level of maturity suitable for commercialization. It enables the creation of a secret key between authorized parties connected by a quantum channel and a classical authenticated channel. The security of the key can be guaranteed in principle without restricting the eavesdropper's capabilities. This review provides an updated, practical overview of QKD, focusing on its theoretical foundations and experimental implementations. The paper begins with an introduction to cryptography and the basics of QKD, including its generic setting, the origin of security, the choice of light, the BB84 protocol, and an example of eavesdropping. It then discusses the elements of practical QKD, including milestones, generic protocols, security notions, and explicit protocols. The review covers discrete-variable, continuous-variable, and distributed-phase-reference protocols, detailing their security proofs, bounds, and practical considerations. The paper emphasizes the importance of theoretical tools in assessing the security of QKD protocols, including the secret key rate, the impact of losses, and the challenges of eavesdropping. It also addresses the practical aspects of QKD, such as the choice of light, sources, physical channels, detectors, and synchronization. The review highlights the differences between discrete-variable and continuous-variable protocols, as well as the challenges of distributed-phase-reference protocols. The paper concludes with a discussion of the future perspectives of QKD, including finite-key analysis, open issues in unconditional security, black-box security proofs, and the potential for longer distances through satellites and repeaters. It also compares different experimental platforms and discusses the importance of practical considerations in the implementation of QKD. The review underscores the need for a balance between theoretical security proofs and practical implementation, highlighting the complexity of comparing different QKD platforms.