Tunable integrated narrow linewidth external cavity semiconductor lasers exhibit many advantages, including small size, wide tuning range, single mode, large side mode suppression ratio, and high output power. Furthermore, they have been widely used in coherent optical communications, wavelength division multiplexing systems, coherent detection, and ultrahigh-speed optical interconnects. This article introduces the development of the operating principle of narrow linewidth semiconductor external cavity lasers in detail and comparatively analyzes the performance and design ideas of integrated external cavity lasers on current mainstream platforms. Finally, we explore the development of tunable narrow linewidth semiconductor lasers based on passive external cavities and discuss their future prospects in the mid-infrared band.