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With high‐temporal coherence and low‐phase noise, narrow‐linewidth lasers have important applications in spectroscopy, holography, and coherent detection. At present, ultra‐narrow‐linewidth lasing is generated by solid‐state, optical fiber, and semiconductor lasers, which serve as a good light source for scientific research and industrial applications. In this article, we summarize the fundamental techniques and review recent developments in narrow‐linewidth lasers. Moreover, typical applications of narrow‐linewidth lasers are discussed to provide reliable and easy‐to‐use references for comparing the performance of narrow‐linewidth lasers.