AbstractWavelength tunable lasers are considered to be used for a wide range of applications such as local oscillator lasers for coherent communication, lasers for wavelength multiplexed communication, and improvement is expected by wavelength matching. So far, various wavelength tunable lasers have been reported. In particular, since diffraction grating integrated lasers such as DBR and DFB lasers can provide wavelength tuning operation by current injection, they have actively been researched and developed. This paper discusses the wavelength tuning principle, structures, and characteristics for these monolithic wavelength tunable lasers.The widest range of wavelength tuning, approximately 10 nm of tunable range, has been obtained by a three‐region DBR laser with a phase control region, but it has a complex structure and there are problems of low light output power and linewidth broadening associated with wavelength tuning. Although any DFB type tunable laser exhibits a small continuous tunable range of at most 2 to 3 nm compared to DBR type, it can provide a narrow linewidth and has an advantage of simple structure. Here, we show that these tuning ranges are theoretical limits, and further improvement in the wavelength tuning range requires the development of new technolog