IN view of the growing interest in spectroscopy at radio and micro­ wave frequencies, and the increasing number of its applications to both physics and chemistry, it was thought that a general outline of the subject for non-specialists might be of some value. Research in this field is still expanding, but is now sufficiently developed for a critical review to be made both of its main applications and of the techniques that are used in this wavelength region. A broad approach has been taken, and the similarity and inter­ relation of the different branches have been stressed, as well as their general setting in spectroscopy as a whole. In this way it is hoped that the book will be of interest to many research workers and students who, although not directly concerned with the subject, would like to obtain a general picture of its methods and applica­ tions. At the same time considerable space has been given to the design of experimental apparatus and equipment, so that those wishing to set up such spectroscopes should be able to find much useful and detailed information.