This chapter presents a summary of the phenomenon of cavitation, covering the conceptualization and characteristic parameters, cavitation caused by irregularities, the protective measures and the specifications for surface protection and several cases of accidents in spillways and tunnels. Cavitation is a dynamic phenomenon consisting of the formation and subsequent collapse of cavities, or vapor bubbles, in a flowing liquid. The onset and intensity of cavitation depend on the dynamic structure of the flow characterized by the velocity distribution, the characteristics of the boundary layer and the mean pressures and pressure fluctuations. The most frequent measure for protection of the concrete surfaces of the discharge organs submitted to free surface flows with high speeds against cavitation erosion caused by irregularities has been the adoption of more stringent specifications for the finishing of these surfaces. In the stretches of the discharge devices with pressure flow, erosion by cavitation can in many cases be avoided by securing a suitable cross-section to the conduit.