A historical introduction and an outline of the radio frequency spectroscopy of the OH molecule is followed by a description of radio frequency absorption by galactic OH and of radio emission from OH in dust clouds. The outstanding features of anomalous radio emission from sources associated with H II regions are described and an account of the interpretation in terms of stimulated emission is given, emphasis being laid on a phenomenological description of the observations. Finally the implications of the observations for the dynamics and constitutions of interstellar gas and for the process going on within it, are considered and the extent of our ignorance of the properties of the OH molecule and the further need for laboratory investigation are indicated. © 1969.
