There are three major revolutionary milestones in the laser field:the invention of solid-state lasers and gas-state lasers in 1960,the invention of bipolar semiconductor lasers in 1962,and the invention of unipolar quantum cascade lasers (QCLs) in 1994.Currently,mid- and far-infrared unipolar QCLs cover 2.63-360 μm wavelength,continuous wave operation at room temperature with power up to watts.This paper gives an overview on the invention of the QCL and research progress in its theory,materials and devices.The article also discusses the current research status and the future development of QCLs and their applications in emerging strategic areas,including environment and climate monitoring,medical imaging and testing for disease diagnosis,communications,and homeland security.