At the beginnning a general description of cavitation phenomena occurring in the hydraulic machinery is given. The purpose and methods of cavitation diagnostics are explained. Exemplary results of authors'  own cavitation studies are presented together with a discussion of relations between different symptoms of cavitation in water turbines and pumps. Prediction of cavitation and cavitation erosion is discussed in a separate chapter. The problem of selection of hydraulic machinery operating parameters from the viewpoint of cavitation is also taken into consideration. Finally ruture goals of cavitation  diagnostics in the hydraulic machinery are shown.