Ultrasonic waves injected into a crystal undergo interactions with the crystal lattice and the lattice vibrations, with conduction electrons and electron magnetic moments and with defects in the lattice. The generation of ultrasonic waves at microwave frequencies has allowed the study of such interactions to be extended to above 1 GHz and has provided data which could previously only be estimated from measurements averaged over the whole spectrum of the thermal vibrations of the lattice. At the same time, investigations of the ultrasonic propagation at these frequencies may lead to useful applications in devices.