Abstract— According to insurance company data, the most expensive failures of air-cooled gas turbine generators have been due to stator winding copper conductor fatigue cracking caused by high levels of endwinding vibration. This has occurred in most brands of turbine generators rated from 100 – 300 MVA, manufactured in the past decade. The root cause is believed to be from inadequate design or imperfect manufacturing that has let the endwinding to be insufficiently supported. Excessive levels of endwinding vibration are most easily detected using fiber-optic accelerometers installed at specific locations in the endwinding. Over the past few years experience has been gained on the collection, display and interpretation of stator winding on-line endwinding vibration data. Although it has been traditional to evaluate results in terms of either total displacement (across the measured frequency range) or the displacement at a specific frequency such as 100 or 120 Hz, it seems that it is sometimes useful to also display the data in terms of velocity. This enables the better detection of vibrations caused by power frequency harmonics or the higher frequencies that result from impacting of loose endwinding components. Enough data has also been accumulated to suggest “alert” levels for high endwinding vibration.