Terahertz technology has moved on from being a useful but expensive circuit technique, applied largely in astronomy and space science, to become a subject in its own right, with important applications - terahertz imaging in particular. Indeed, the driving force in terahertz technology is currently imaging and spectroscopy. We now have the means to obtain images and chemical information in this frequency band. The images reproduced in this volume are striking and, not surprisingly, the clinical and analytical uses are the subject of intense activity. There is still, however, no complete range of active THz electronic components, but an encouraging conclusion of the book is that THz electronics will become necessary in communications systems in the foreseeable future. Terahertz technology has come of age, and the future lies open to new, exciting science and vital applications.