An explanation is first given of wear-monitoring diagnostic methods that depend on the interpretation of symptoms rather than direct wear. These methods include thermography and oil temperature measurements, which are checks for friction; oil analysis, which can determine metal loss; and vibration analysis, which can identify the source of a problem but cannot accurately measure bearing wear. Other online methods that directly measure wear and eliminate costly downtime from routine disassembly are then discussed, including ultrasonic transducers, eddy-current proximity transducers, and surface layer activation.