Terahertz waves, which lie in the frequency range of 0.1-10 THz, have long been investigated in a few limited fields, such as astronomy, because of a lack of devices for their generation and detection. Several technical breakthroughs made over the last couple of decades now allow us to radiate and detect terahertz waves more easily, which has triggered the search for new uses of terahertz waves in many fields, such as bioscience, security, and information and communications technology. The book covers some of the technical breakthroughs in terms of device technologies. It discusses not only the theoretical details and typical features of the technology described, but also some issues and challenges related to it. In addition, it is shown what can actually be done with the terahertz-wave technologies by introducing several successful demonstrations, such as wireless communications, industrial uses, remote sensing, chemical analysis, and 2D/3D imaging.