Many exciting new developments in microwave spectroscopy have occurred since the earlier edition of this book was published in 1970. The frequency coverage of coherent microwave spectroscopy is still expanding. Although not comparable to the fourfold expansion that occurred in the 17 years preceding the 1970 edition, the millimeter wave range has been extended from 800 GHz to above 100 GHz since 1970. Subtle new techniques continue to improve the sensitivity and resolving power of microwave spectrometers as well as their applicability to new types of spectra. Although the spectra described in the earlier edition are in no sense out of date, the new developments made the revision of Microwave Molecular Spectra desirable if not necessary. Because the authors could not justifiably delete nor significantly reduce the basic material of the earlier edition, a moderate expansion of the volume was necessary to achieve an adequate coverage of microwave molecular spectra in the 1980's.