Part I. Bandstructure Engineering, Modeling and State-of-the-art QCLs: 1. Basic physics of intersubband radiative and nonradiative processes Jacob B. Khurgin; 2. State-of-the-art mid-infrared QCLs: elastic scattering, high CW power and coherent-power scaling Dan Botez and Luke J. Mawst; 3. Long wavelength mid-infrared quantum cascade lasers Alexei Baranov, Michael Bahriz and Roland Teissier; 4. Overview of the state-of-the-art terahertz QCL designs Qi Jie Wang and Yongquan Zeng; 5. Simulating quantum cascade lasers: the challenge to quantum theory Andreas Wacker; 6. Coupled simulation of quantum electronic transport and thermal transport in mid-infrared quantum cascade lasers Michelle L. King, Farhad Karimi, Sina Soleimanikahnoj, Suraj Suri, Song Mei, Yanbing Shi, Olafur Jonasson and Irena Knezevic; Part II. Active Research Topics: 7. Quantum cascade laser frequency combs Jérôme_Faist and Giacomo Scalari; 8. Frequency noise and frequency stabilization of QCLs Miriam Serena Vitiello, Luigi Consolino and Paolo De Natale; 9. Distributed-feedback and beam shaping in monolithic terahertz QCLs Yuan Jin and Sushil Kumar; 10. Metasurface based THz quantum-cascade lasers Benjamin S. Williams and Christopher A. Curwen; 11. Terahertz quantum cascade laser sources based on intra-cavity difference-frequency generation Mikhail A. Belkin; Part III. Applications: 12. QCL applications in scientific research, commercial, and defense and security markets Jeremy Rowlette, Eric Takeuchi and Timothy Day; 13. QCL-based gas sensing with photoacoustic spectroscopy Vincenzo Spagnolo, Pietro Patimisco, Angelo Sampaolo and Marilena Giglio; 14. Multiheterodyne spectroscopic sensing and applications of mid-infrared and terahertz quantum cascade laser combs; Gerard Wysocki, Jonas Westberg and Lukasz Sterczewski; 15. Self-mixing in quantum cascade lasers: theory and applications Paul Dean, Jay Keeley, Yah Leng Lim, Karl Bertling, Thomas Taimre, Pierluigi Rubino, Dragan Indjin and Aleksandar Rakic; 16. Applications of terahertz quantum cascade lasers Pierre Gellie; Index.