The diagnostic examination of partial discharges (PD) is regarded a powerful, non-destructive and sensitive diagnostic tool for the condition assessment of insulating systems. In order to judge the insulation condition of a transformer general complete information about a PD activity comprise its level, its type and location. The ’non-conventional’ electromagnetic (UHF) and acoustic PD measuring methods have advantages for the required on-site/on-line applications. Due to the sensitivity differences between the UHF and the acoustic signals a combination of the two methods is reasonable regarding the de-noising of acoustic signals. Particularly for answering the question “Where is the PD source?”, which is of great interest for finding the reason for the PD activity and risk assessment, the inherent possibility of the acoustic method to geometrically localize the flaw by means of signal propagation delays is an additional benefit. The three dimensional determination of the PD site is finally managed using acoustic signals in a mixedacoustic (i.e. electromagnetic triggering) or all-acoustic acoustic triggering) manner together with enhanced signal processing techniques and new robust positioning algorithms.