A gearbox condition monitoring system comprises at least one vibration sensor (74,76,78,80) located on a gearbox casing (24). A processing unit (82) is coupled to the vibration sensor and is configured to receive signals representative of the detected vibrations from the vibration sensor. The processing unit may be operable to process the signals representative of detected vibrations of gears (18,20,22) and bearings (34,36,etc.) and to compute at least one dynamic energy index or location of fault.