Dissolved Gas Analysis is one of the best known methods to detect incipient faults in power transformers. Characteristic gases for power transformers are defined in IEC 60599. However, these gases migrate between the transformer liquid and solid insulation due to temperature variations; this could potentially cause errors in the fault detection method. Little analytical or experimental work has been performed to date on the temporal nature of this problem. The suggested dynamic model in this book is based on the process of diffusion. The steady-state information extracted from experiments is compared with findings obtained from the literature. The final dynamic model is tested against experiments. The results show that the migration phenomenon in the liquid-solid insulation of power transformers can be explained by diffusion equations. Other findings include diffusion coefficients and steady- state information of the characteristic gases in the transformer insulation system. This text is suitable for researchers who work in the area of fault detection in high voltage fluid-filled equipment or students who are interested in mass transport phenomena.