The external cavity diode laser (ECDL) uses the LD chip as gain element and external reflectors as cavity mirrors, instead of the cleaved facets of LD chip. The cavity length can then be extended for a longer photon lifetime; and the external reflector may have strong wavelength selectivity to suppress mode competition effectively. The external reflector can provide an additional tuning method. In this Chapter, general characteristics and theory of ECDL are explained in Sect. 3.1. ECDLs incorporated with planar diffractive gratings are described in Sect. 3.2, especially the structures of Littrow type and Littman type. Section 3.3 introduces ECDL with Bragg gratings, including the fiber Bragg gratings (FBG), the waveguide Bragg grating (WBG) and the volume Bragg grating (VBG). Section 3.4 focuses on ECDL with external Fabry-Perot resonator and ring resonator.