The invention of quantum cascade lasers was a milestone in the semiconductor laser field, and created a new field of semiconductor laser for operation in mid-infrared to far-infrared. Quantum cascade lasers are playing important roles in many fields, such as:infrared countermeasures, drug and explosive detection, environment survey and terahertz imaging. In this paper, we described the theory,the research on materials and devices of quantum cascade lasers. We also gave an overview on research progress and applications of quantum cascade lasers.