Designed for practitioners in both industry and academia, this book demonstrates the current status and future potential of millimetre wavelength (MMW) spectrometry as a means of quantitative analysis of gaseous mixtures. After outlining the spectroscopic theory required, the authors discuss the components of an MMW cavity spectrometer, concentrating on compact, automatic, low-cost instruments. They then cover solid state MMW sources with cryogenically cooled and room temperature detectors, post-detector signal processing, smoothing, filtering and spectral profile fitting. The book concludes with a look at the future of the technique, in areas such as millimetre wave-over-fibre technology.