Several interesting phenomena involving the interaction of ultrasonic waves and electrons have been observed in piezo-electric semiconductors. These include the processes of ultrasonic amplification and nonlinear harmonic generation, which are manifestations of the action of electrons on ultrasonic waves. The reaction upon the electrons as a result of their strong coupling to ultrasonic waves is manifested by the phenomena of current saturation and current oscillations in amplifying semiconductor crystals. We discuss the principle of acoustoelectric amplification and the role it plays in these effects. The difference between the amplification process and the Cerenkov emission of sound waves is emphasized. The generation of harmonics of the sound wave due to the nonlinear transport properties of the electrons is reviewed. Various models of current saturation are considered, including an elementary treatment of the role of acoustoelectric fields. A discussion is given of the possibility of propagating collective sound waves under amplifying conditions and the role that this may play in the current oscillations in cadmium sulphide.