Cavitation is a very effective source of noise. As soon as it appears one observes a marked rise in the level of noise emitted and vibrations experienced by the hydraulic components and circuits. This specific characteristic of the phenomenon is used for detecting its appearance. As soon as a very small number of vapour bubbles appear, the level of the noise radiated rises suddenly. As soon as noise of high frequency, typically higher than 20 kHz appears from a flow, one can, without risk of error, be certain that its source is cavitation even if one cannot visualise it. The studies on noise reduction in military vessels, especially submarines, have generated, in a number of laboratories, interest in the phenomenon and development of theoretical and experimental methods for detecting and predicting it.