Abstract: Quantum cascade laser (QCL) has been widely applied in directed infrared countermeasures (DIRCM) system, free space optical communication (FSOC) and gas sensing since it has advantages of high efficiency, compact volume, low electrical consumption and wide wavelength tunability. In this paper, the progress in QCLs made over the last 20 years was reviewed. The principle of emission and overview of QCL was demonstrated in the introduction. The design of active region for high output power QCL aiming at DIRCM was described in the first part. In the second part, the progress in distributed-feedback QCLs for gas sensing was introduced. In the third part, the research of high brightness QCL phase locked arrays was demonstrated. In the fourth part, the high speed QCL for FSOC was discussed. Finally, a new device, QCL frequency comb was introduced for its crucial influence on mid-infrared frequency comb.