Diagnostic tests and measurements that are performed in the field on fluid-filled power transformers and regulators are described. Whenever possible, shunt reactors are treated in a similar manner to transformers. Tests are presented systematically in categories depending on the subsystem of the unit being examined. A diagnostic chart is included as an aid to identify the various subsystems. Additional information is provided regarding specialized test and measuring techniques. Interpretive discussions are also included in several areas to provide additional insight on the particular test or to provide guidance on acceptance criteria. These discussions are based on the authors’ judgment of accepted practice. It should be noted that the results of several types of tests should be interpreted together to diagnose a problem. Manufacturers’ acceptance criteria should also be consulted as it may take precedence over the criteria in this guide.