Discover how mid-infrared and terahertz photonics has been revolutionized in this comprehensive overview of state-of-the art quantum cascade lasers (QCLs). Combining real-world examples with expert guidance, it provides a thorough treatment of practical applications, including high-power continuous-wave QCLs, frequency-comb devices, quantum-electronic transport and thermal transport modeling, and beam shaping in QCLs. With a focus on recent developments, such as frequency noise and frequency stabilization of QCLs, grating-outcoupled surface-emitting mid-infrared QCLs, coherent-power scaling of mid-IR and THz QCLs, metasurface-based surface-emitting THz QCLs, self-mixing in QCLs, and THz QCL sources based on difference-frequency generation, it also features detailed theoretical explanations of means for efficiency maximization, design criteria for high-power continuous-wave operation of QCLs, and QCL thermal modeling, enabling you to improve performance of current and future devices. Pa