The scope of microwave spectroscopy and its relationship to other branches of spectroscopy are briefly indicated, attention being especially directed to those aspects which are important in the design of spectrometers. The factors determining spectrometer sensitivity are discussed. The basis of molecular modulation is presented with special reference to Stark modulation. A survey is made of the various types of spectrometer and their relative merits are discussed. The application of the method to chemical analysis is considered and indications are given of the directions in which further experimental work is required.