Secure communication is crucial in the Internet Age, and quantum mechanics offers a revolutionary approach to cryptography. Quantum Key Distribution (QKD) promises unconditional security, ensuring that Eve's actions are limited by the laws of physics rather than computational assumptions. This review introduces the motivation and current state of quantum cryptography, focusing on the security model, its assumptions, strengths, and weaknesses. It discusses recent experimental progress and challenges, including quantum hacking and counter-measures. QKD, particularly the BB84 protocol, has achieved stable operation over long distances using both fibers and free-space. Commercial QKD systems are available, and field tests have demonstrated the feasibility of QKD networks. Key achievements include rigorous security proofs, high detection efficiency single-photon detectors, and the development of decoy-state methods to improve key generation rates. However, practical QKD implementations face challenges such as device imperfections and the need for multiplexing strong classical signals with weak quantum signals. Researchers are working on high-speed QKD systems and extending QKD coverage to longer distances. The concept of ground-to-satellite QKD is also under investigation. Quantum hacking attacks, such as detector blinding and time-shift attacks, have been demonstrated against commercial and research QKD setups. Counter-measures include security patches, device-independent (DI) QKD, and measurement-device-independent (MDI) QKD. MDI-QKD, which uses untrusted measurement devices, is a promising solution that can be implemented with current technology and offers higher key rates than DI-QKD. The future of QKD includes the development of quantum repeaters and the potential deployment of MDI-QKD, which could bridge the gap between theoretical and practical security. Quantum cryptography also has deep connections with other areas of physics, mathematics, and technology, driving advancements in single-photon detectors and quantum metrology.Secure communication is crucial in the Internet Age, and quantum mechanics offers a revolutionary approach to cryptography. Quantum Key Distribution (QKD) promises unconditional security, ensuring that Eve's actions are limited by the laws of physics rather than computational assumptions. This review introduces the motivation and current state of quantum cryptography, focusing on the security model, its assumptions, strengths, and weaknesses. It discusses recent experimental progress and challenges, including quantum hacking and counter-measures. QKD, particularly the BB84 protocol, has achieved stable operation over long distances using both fibers and free-space. Commercial QKD systems are available, and field tests have demonstrated the feasibility of QKD networks. Key achievements include rigorous security proofs, high detection efficiency single-photon detectors, and the development of decoy-state methods to improve key generation rates. However, practical QKD implementations face challenges such as device imperfections and the need for multiplexing strong classical signals with weak quantum signals. Researchers are working on high-speed QKD systems and extending QKD coverage to longer distances. The concept of ground-to-satellite QKD is also under investigation. Quantum hacking attacks, such as detector blinding and time-shift attacks, have been demonstrated against commercial and research QKD setups. Counter-measures include security patches, device-independent (DI) QKD, and measurement-device-independent (MDI) QKD. MDI-QKD, which uses untrusted measurement devices, is a promising solution that can be implemented with current technology and offers higher key rates than DI-QKD. The future of QKD includes the development of quantum repeaters and the potential deployment of MDI-QKD, which could bridge the gap between theoretical and practical security. Quantum cryptography also has deep connections with other areas of physics, mathematics, and technology, driving advancements in single-photon detectors and quantum metrology.