Abstract – Many end users, manufacturers and service organizations now routinely employ some form of on-line monitoring technology, e.g., partial discharge (PD), as a significant component of a condition based maintenance program. The widespread and increasing use of such methods has led to some organizations questioning the value of performing off-line diagnostic testing to assess machine insulation condition. Traditionally, many end users would do one or more of these tests during major outages, however, due to economic pressures the interval between such outages has significantly increased, in many cases by a factor of two. Further, many plant managers are reluctant to permit electrical testing because of concerns that such testing may damage the machine even though the applied voltage used in the vast majority of tests is limited to the nominal line-to-ground operating voltage. In this contribution, we shall discuss the role that off-line tests have to play in the testing of rotor windings, stator windings and cores, the commonly used tests and their appropriate application. Only those tests that are covered by IEEE or IEC standards are discussed.