Based on the intersubband transitions of multiple quantum wells, the quantum cascade laser (QCL) can be considered to be an unipolar semiconductor laser. The lasing frequency can be observed in the mid-infrared, far-infrared, and terahertz regions. Terahertz QCL is the most effective electrically pumped terahertz radiation source in the frequency range of 1--5 THz and exhibits the advantages of compactness, easy integration, high output power, and high conversion efficiency. In this study, the active regions, waveguide structures, and materials associated with terahertz QCLs are initially described. Next, the high-performance terahertz QCLs are presented under the conditions of extreme lasing frequency, high operating temperature, and high output power. Subsequently, the recent advances in terahertz QCLs are reviewed from the aspects of photonic engineering, including one-dimensional gratings, two-dimensional photonic structures, and metasurfaces. Additionally, the progress in case of frequency combs based on terahertz QCLs in our laboratory, such as active-frequency combs, passive-frequency combs, and terahertz dual-combs, is reviewed.