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With the potential of quantum computers to compromise current public-key cryptosystems, Quantum Key Distribution (QKD), which is based on the physical laws of quantum mechanics, has garnered significant attention. However, while QKD security proofs typically assume ideal single-photon sources, realistic implementations rely on Weak Coherent Pulses (WCP) due to the difficulties associated with commercializing true single-photon sources. Unfortunately, the multi-photon components inherent in WCPs are vulnerable to Photon Number Splitting (PNS) attacks, which limits the secure transmission distance to a few tens of kilometers. This article introduces the Decoy State method, which effectively resolves this issue. Proposed by W.-Y. Hwang in 2003 and further advanced by H.-K. Lo and X.-B. Wang, this technique utilizes the random interleaving of pulses with different mean photon numbers during transmission. By doing so, it enables the accurate estimation of single-photon yield and error rates, thereby effectively detecting PNS attacks. Because it can be implemented using standard commercial lasers and intensity modulators, the Decoy State method has become the standard technology in most modern QKD systems. It serves as a core component for long-distance quantum communication, enabling major milestones reported recently. This article reviews the principles, mathematical analysis, and latest applications of the Decoy State method.