ZnO varistors are novel ceramic semiconductor devices used for protection of electronic equipment against transient overvoltages due to switching surges or lightning. This article describes the microstructure, circuit behavior, and operation of ZnO varistors. ZnO varistors have highly nonlinear current-voltage characteristics similar to back-to-back Zener diodes but with much greater current, voltage, and energy handling capability. These devices are capable of protecting circuits with operating voltages from ten to thousands of volts while passing surge currents in the kiloamp range. Varistor response time is too small to measure (less than 500 ps). ZnO varisters are comprised of an array of conducting ( \Omega ·cm) ZnO grains, each grain being surrounded by an insulating oxide barrier 106V/cm) across the insulating barriers promote the tunneling of electrons through these intergranular barriers. The details of the varistor conduction mechanism are complex and a discussion is given of our present understanding of this process.