Abstract: The authors dedicate this paper to the memory of Hui-Chun Liu—the renowned physicist and world leader in the area of intersubband transitions in quantum wells and dots. We review the research on terahertz (THz) semiconductors and their applications which was carried out by the authors in collaboration with H. C. Liu. The past decade has seen a revolution in THz science and technology, and the potential of THz radiation for applications in a vast range of disciplines has been demonstrated. We report the latest progress in the development and application of THz semiconductor devices, including oscillators based on negative effective mass, sources based on semiconductors pumped by femtosecond lasers, quantum cascade lasers, quantum well photodetectors, and communication and imaging applications, all in the THz regime.