Mid‐infrared spectroscopy (MIR) has been applied for decades in a wide range of applications, since it is a nondestructive and versatile analytical technique allowing for the analysis of chemically and biologically relevant compounds with high sensitivity and selectivity. Laser spectroscopy provides the most advanced technological approach in IR spectroscopy with several benefits compared to conventional infrared light sources. This article highlights the fundamentals of the latest generation of IR lasers, i.e. so‐called cascade lasers. Among their advantages are their high output power, the narrow linewidth, operation in continuous or tailorable pulse mode, reliability during long‐term usage, on‐chip dimensions, and tunability. Quantum Cascade and Interband Cascade Lasers (QCLs, ICLs) are nowadays available across almost the entire infrared range extending into the terahertz (THz) regime and may either be tuned over several hundreds of wavenumbers or designed to emit a specific wavelength. The most important classes of cascade lasers to date – ICLs and QCLs – will be discussed in their theoretical and technical fundamentals and working principles. Complementarily, selected applications will be highlighted illustrating the utility of these most advanced IR laser light sources available for modern infrared spectroscopy and sensing.