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This Special Issue is dedicated to the use of THz radiation for sensing. Previously known as the sub-mm or far infrared spectral range, the technological difficulties associated with the fabrication of suitable sources and detectors for THz frequencies has, until recently, limited their use to a small number of laboratories. The availability of elements like frequency multiplier stages and photo-conductive switches, to name only a couple, has now permitted a wider scientific and engineering community to pursue a vast range of objectives using THz radiation. Many of the contributions deal with the development and application of complete systems, identifying the fine details of how the potential of THz waves can be harnessed. Approaching the THz domain from a bottom-up rather than a top-down point of view, this Special Issue is also rich in the latest device developments. The highlights include the demonstration of a transistor able to be used as a sub-harmonic mixer in the THz frequency band. This is accompanied by a paper showing that quantum cascade lasers are not only useful sources but can also simultaneously be used as efficient detection devices. The papers contained in this Special Issue show that the THz community is particularly active undertaking a wide range of investigations related to sensing principles and devices. Although THz radiation has been proposed to solve a large range of problems it is not a panacea. It does, however, have some unique properties, and so it has a role to play in satisfying the needs of ever more sophisticated sensing tasks. The future development of THz waves for sensing will without doubt be thanks to the mutual stimulation provided by workers developing new components, gaining a further understanding of physical processes at THz frequencies, and demonstrating applications unique to this domain.