Phonon spectroscopy is the use of very high frequency sound at low temperatures to investigate ions or defects in solids and at surfaces. The sound frequencies extend up to 1000 GHz and descriptions are given of how this sound is generated and detected, and how it is used in spectroscopy. These methods are illustrated by the results of recent experiments carried out in a number of laboratories. Some of the systems which can readily be detected using sound are very difficult to see by other methods and the reasons for this large difference in sensitivity are discussed.