Mid-infrared to terahertz spectral regions (3~300 μm) are of particular interest to sensing and monitoring since a large number of gases such as CO, CH₄, NH₃, SO₂, HCl, etc., has strong characteristic vibrational transitions. These molecular fingerprints enables fast and pricise detection when a high power laser and a sensitive detector is applied. A morden sensing system normally includes a separate laser, a detector, and a gas chamber, which makes the system rather bulky and expensive. A chip-based sensing system on the other hand, with all the components integrated on a single chip, will greatly reduce the size, weight, power, and cost of the system. Currently, there are limited light source candidates for mid-infrared and terahertz sprectral bands. Devices that can generate light sources in the infrared band include: quantum well lasers, solid-state laser-pumped optical parametric oscillators, interband cascade lasers, and quantum cascade lasers. As a conventional semiconductor laser light source, quantum well laser is based on the optical transition between the conduction band electrons and valence band holes in the quantum wells, its emitting wavelength is determined by the band gap of the semiconductor quantum material. The photon energy in the mid- and far-infrared bands ranges from 0.04~0.4 eV, which is much smaller than the bandgap of the majority of quantum well laser materials themselves. It is difficult for traditional quantum well lasers to cover the mid- and far-infrared as well as terahertz bands. Devices using solid-state laser-pumped optical parametric oscillators to generate the mid- and far-infrared emission are generally large in size and low in efficiency, and have limitations in practical applications. In addition, based on cascade transition between the conduction band and the valence band, interband cascade lasers can cover the 3~6 μm band. These type of laser is of much lower threshold current density and threshold power density. However, for the mid-infrared and even further wavelength bands, the performance of interband cascade lasers shows a decreasing trend. Based on intersubband transition in the conduction bands, Quantum Cascade Laser (QCL) is a new type of semiconductor laser source with high responsivity, high nonlinearity, and wide spectral coverage from mid-infrared to terahertz band. Compared with other laser sources that can produce mid-infrared sources, quantum cascade lasers have the advantages of small size, high energy efficiency, and wider wavelength tunability, and their wavelength ranges have been extended to the 3~25 μm and 1~6 THz bands, which make them the most promising laser light sources in the mid-infrared band.In recent years, quantum cascade lasers have important applications in the fields of long-range hazardous and explosive detection, biomedicine, infrared countermeasures and long-range free-space optical communications. Since the sensitivity and detecting range are proportional to the output power of the lasers, further improvement of the output power and electro-optical conversion efficiency of the devices is one of the most important goals of QCL research. By improving the device structure design, material epitaxial growth technology, and device preparation process, the output power, electro-optical conversion efficiency, beam quality, threshold current density, and other key parameters of quantum cascade lasers have been continuously optimized. In addition, broadly tuned, single-mode quantum cascade lasers with high side-mode suppression ratio have also become a research hotspot due to their urgent needs in gas sensing, spectroscopic measurements and other fields. Further, difference-frequency generation terahertz light sources and on-chip sensing technologies based on quantum cascade lasers are attracting more and more attention for the potential applications.With the recent rapid development in output power and wall-plug efficiency, QCL is becoming the most promising laser source in mid-IR and THz regions. The high power characteristic together with the strong absorption at zero bias makes QCL the ideal platform for on-chip sensing. This paper focuses on the recent development mid-infrared quantum cascade lasers and their application in on-chip sensing. Firstly, the history and working principle of quantum cascade lasers is introduced. Subsequently, the research progress of mid-infrared quantum cascade lasers in wall-plug efficiency, output power, wavelength tunability, single-mode operation, optical frequency comb, THz difference-frequency generation are summarized and their application in on-chip sensing are also discussed.