The analysis of gases dissolved in oil has been used as a diagnostic tool for many years to determine the condition of transformers. The criteria used in evaluating dissolved gas-in-oil data are reviewed. The criteria are based on experience from failed transformers, transformers with incipient faults, laboratory simulations, and statistical studies. The interpretation of results is enhanced by including specific information on a particular transformer, including its history. Comparison of a particular transformer of concern with a population of similar transformers provides additional information as regards “normal” gassing behavior. The interpretation of dissolved gas-in-oil data is not a simple matter and requires the integration of numerous criteria. Diagnostic gases for silicone fluids are also discussed.