The terahertz frequency region of the electromagnetic spectrum lies between the lower frequency electronic and the higher frequency photonic regions. It is an area of vast scientific and technological interest and potential but is relatively unexplored due to the technical challenges involved in producing efficient, inexpensive, robust, compact, and reliable sources and detectors of terahertz radiation. Terahertz imaging is accomplished by raster scanning or by using focal-plane arrays. Terahertz tomography has also been demonstrated. Terahertz spectroscopy is accomplished using either tunable sources or broadband sources and dispersive spectrometers. The recent development of time-domain terahertz spectroscopy is revolutionizing the field.