Experiments which use coherent waves at frequencies near 1010Hz are possible in suitable materials at low enough temperatures. The techniques which are used are determined by the very short acoustic wavelength (̃ 5000 Å) and by the availability of transducers and detectors. An interesting area of research is into their use for spectroscopic purposes in crystals, a technique which is complementary to electron spin resonance (e.s.r.). As the selection rules and the quantities which govern the intensities are quite different, it is possible to detect impurities which e.s.r. misses. By obtaining information on the strength of the impurity-lattice vibration coupling a good deal of light is thrown on the problem of spin-lattice relaxation. The propagation of pulses in the vicinity of absorption lines gives some striking demonstrations of anomalous propagation. A related technique is that in which the waves, are generated incoherently from a black body source. This extends the range of frequencies which can be used, though at the expense of increased difficulty in the interpretation of the results. Nevertheless, some remarkable changes in the thermal conductivity of crystals at low temperatures can be obtained in a magnetic field. The general field is one of considerable promise and some possible directions in which developments may occur are outlined.