This guide provides background information and general recommendations of instrumentation@ circuitry@ calibration@ and measurement techniques of no-load losses (excluding auxiliary losses)@ excitation current@ and load losses of power and distribution transformers. The test codes@ namely@ IEEE Std C57.12.90TM@ IEEE Std C57.12.91TM@ and the test code section of IEEE Std C57.15TM@ provide specifications and requirements for conducting these tests. This guide has been written to provide supplemental information for each test. More technical details of the measuring instruments and techniques presented in this guide can be found in the document developed by So [B16]. This guide applies to liquid-immersed-power and distribution transformers@ dry-type transformers@ and step-voltage regulators. Additionally@ it applies to both single- and three-phase transformers.