The optically pumped laser has made an enormous contribution to research in the part of the electromagnetic spectrum known as the far infrared, or submillimetre region. I hope that this book will be useful to both practising and prospective workers in the field, since it contains an up-to-date catalogue of measurements of the main properties of submillimetre lasers as well as an introductory review of the measurement techniques themselves. Wavelength and frequency measurements have been exhaustively compiled (in Part II of this book) along with molecule and pump identification. Part I contains a short review of the relevant measurement techniques in each of these areas and, in addition, a review of power measurements. Working in this field, as in any other, one's satisfaction is determined largely by the colleagues one has and the friends one makes along the way. I am very grateful to Dr G. Dodel, Dr L. C. Robinson and Dr G. F. Brand for introducing me to the field. Dr I. S. Falconer and Dr P. A. Krug have been good colleagues and friends. For this book in particular I am grateful to Dr Dodel, Dr K. M. Evenson, Dr H. Figger, Prof. M. Fourrier, P. Kempf, Dr K. J. Siemsen and Dr M. S. Tobin for their comments, and to Dr D. J. E. Knight for a great deal of help, including data from unpublished or obscure sources which he had gathered for his own compilation.