The acoustic emission monitoring technique has demonstrated a wide range of applications in the detection of material and equipment failures. Thus, it has been used for several years for the detection and location of Partial Discharges inside dry and wet high power transformers. We at MATGEO SpA. Have been able to measure these PD phenomena with great precision in transformers of both types, being able to locate the damaged element inside these equipments, thus helping predictive maintenance in large and costly components.