Gas Insulated Substations (GIS) have obtained great popularity in the world by their high reliability, compactness, easy maintenance, and environmental compatibility. In GIS, all live parts such as conductors, circuit breakers, disconnectors are totally encapsulated by metal enclosures. Such systems make the visual inspection of live parts problematic and enhance the importance of suitable diagnostic tools. Among the various diagnostic methods, partial discharge detection, wich plays the most important role, is analyzed in this paper. It is shown that Partial Discharge (PD) measurements according to standard methods can be used to detect `critical` GIS defects in the laboratory: they are however of difficult application and may result not sensitive enough for `on site` application. It is also shown that alternative measurements, namely: acoustical, electrical with probes located inside the GIS, and chemical are more practical and offer adequate `on site` sensitivity.