The article provides a comprehensive review of practical Quantum Key Distribution (QKD) technology, focusing on its theoretical foundations, experimental progress, and security aspects. QKD aims to create a secret key between authorized parties over a quantum channel and a classical authenticated channel, with the security guaranteed without restricting the eavesdropper's power. The review covers the evolution of QKD from its early theoretical proposals to its current state of practical implementation, highlighting key milestones and advancements. Key sections include: 1. **Introduction**: Discusses the history of cryptography, the development of QKD, and the importance of light as the medium for QKD. 2. **The Elements of Practical QKD**: Explains the basic elements of QKD, including the generic setting, the role of classical and quantum channels, and the importance of authentication. 3. **Secret Key Rate**: Analyzes the rate at which a secret key is produced, considering both the speed and efficiency of devices and the intrinsic security of the protocol against eavesdropping. 4. **Explicit Protocols**: Details three main families of protocols—discrete-variable, continuous-variable, and distributed-phase-reference—providing explicit formulas and security proofs. 5. **Sources, Physical Channels, and Detectors**: Discusses the sources of quantum signals, the types of physical channels, and the detectors used in QKD. 6. **Synchronization and Alignment**: Covers the importance of synchronization and alignment in QKD systems. 7. **Comparison of Experimental Platforms**: Compares different experimental platforms based on their performance in terms of secret key rate, achievable distance, and cost. 8. **Perspectives**: Discusses future directions for QKD, including finite-key analysis, unconditional security, black-box security proofs, and the integration of QKD into networks. The article emphasizes the importance of practical considerations in QKD, such as the choice of light sources, the impact of losses, and the need for rigorous security proofs. It also highlights the ongoing challenges and advancements in the field, providing a detailed and up-to-date overview of QKD technology.The article provides a comprehensive review of practical Quantum Key Distribution (QKD) technology, focusing on its theoretical foundations, experimental progress, and security aspects. QKD aims to create a secret key between authorized parties over a quantum channel and a classical authenticated channel, with the security guaranteed without restricting the eavesdropper's power. The review covers the evolution of QKD from its early theoretical proposals to its current state of practical implementation, highlighting key milestones and advancements. Key sections include: 1. **Introduction**: Discusses the history of cryptography, the development of QKD, and the importance of light as the medium for QKD. 2. **The Elements of Practical QKD**: Explains the basic elements of QKD, including the generic setting, the role of classical and quantum channels, and the importance of authentication. 3. **Secret Key Rate**: Analyzes the rate at which a secret key is produced, considering both the speed and efficiency of devices and the intrinsic security of the protocol against eavesdropping. 4. **Explicit Protocols**: Details three main families of protocols—discrete-variable, continuous-variable, and distributed-phase-reference—providing explicit formulas and security proofs. 5. **Sources, Physical Channels, and Detectors**: Discusses the sources of quantum signals, the types of physical channels, and the detectors used in QKD. 6. **Synchronization and Alignment**: Covers the importance of synchronization and alignment in QKD systems. 7. **Comparison of Experimental Platforms**: Compares different experimental platforms based on their performance in terms of secret key rate, achievable distance, and cost. 8. **Perspectives**: Discusses future directions for QKD, including finite-key analysis, unconditional security, black-box security proofs, and the integration of QKD into networks. The article emphasizes the importance of practical considerations in QKD, such as the choice of light sources, the impact of losses, and the need for rigorous security proofs. It also highlights the ongoing challenges and advancements in the field, providing a detailed and up-to-date overview of QKD technology.