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...