Model testing of hydraulic turbines, storage pumps and pump-turbines is of great importance especially for large units involved in power generation. Among all the aspects of machine operation, cavitation plays a fundamental role with respect to the possible alteration of the efficiency and to the erosion risk. This paper describes the procedures of cavitation tests and the related instrumentation, depending both on the type of hydraulic machine and its operation range. Discussion is mainly directed toward the unavoidable distortions of the similitude requirements between model and industrial scales.