Narrow linewidth lasers have extremely high spectral density, low relative intensity noise, phase noise, and ultra-long coherent length, so they are used as core light sources in many important applications. In this paper, firstly, the basic theory and design ideas of narrow linewidth lasers for reducing linewidth are introduced. Then, the basic structures, key technologies, performance characteristics, and international development status of the four most representative types of narrow-linewidth external cavity diode lasers are introduced. Finally, the application and promotion potential of several types of external cavity lasers are summarized, and the development prospects of narrow linewidth lasers are prospected.