The use of dissolved gas analysis (DGA) to monitor the in-service behavior of transformers is discussed. Sampling techniques are briefly considered, and two commercial hydrogen-in-oil detectors are described. The first allows the hydrogen concentration to be measured at intervals of a few hours by a portable gas collector that can be connected to semipermeable tubes. Continuous remote monitoring from the substation is possible with the second model, which uses a fuel-cell-type detector. The use of DGA for fault diagnosis is examined, and acceptable gas levels are indicated. The use of expert systems to facilitate decision making on the basis of DGA results is discussed, as is international cooperation in sharing data and experience and reaching agreement on methods of analysis and interpretation. Further applications of DGA are indicated. >