Power transformer field drying is becoming a habitual practice because water is damaging for transformer insulation, and its presence increases the probability of unexpected failures. Different drying methods are currently being used by electrical companies but sometimes without a profound knowledge of the drying processes involved, and consequently drying is not as effective as would be desirable. Physical models have been developed by the authors in order to study power transformer drying processes. The use of the models will help to plan more effective drying processes, tailoring the drying times and drying conditions for each particular case. The models have been tested in a test transformer fitted with sensors. In order to monitor the process, insulation samples were extracted from the transformer before and after the process. In this article, several transformer drying procedures are described. Theoretical models and their experimental validation processes are reported. Finally, some general recommendations about transformer drying in the field are provided.