This paper discusses the microwave and millimeter wave fundamentals and applications of high-temperature superconductors. First, a high-temperature superconducting filter subsystem for a base station is described as an example of microwave devices. In the USA, over 1, 000 systems are already working as filter subsystems in 2 G mobile telephone base stations. In Japan, a field trial was carried out for a filter subsystem of an IMT-2000 base station. As the result, the effectiveness of the superconducting filter subsystem was clarified. Therefore, the demand for superconducting filter subsystems for base stations will increase in the future. A SIS mixer and superconducting calorimeter were described as examples of millimeter and sub-millimeter wave devices. Using a Nb/AlOx/Nb SIS junction and X-ray calorimeter, which utilized micromachine technology, the resolution of such devices was quite high compared to that of conventional devices.