In most cases the helicopter transmission system comprises of the main gearbox (MGB), auxiliary gearbox (AGB), intermediate gearbox (IGB) and tail rotor gearbox (TGB). A local gear fault will impose a force variation in the gearbox and changes the gear angular velocity resulting in frequency modulations, which in turn generates sidebands and changes the vibration signature. The change in vibration signature contains information about the health of the gearbox from which diagnosis can be made to prevent the catastrophic effect of propagated fault. The helicopter gearbox vibration mode differs from those of other systems due to the transmission noise, structural noise and aero acoustic noise which masks the sideband. Thus an attempt is made to review the condition indicators that have been applied for fault diagnosis on the helicopter gearbox. This review is intended to advance the knowledge and the performance of Health and Usage Monitoring System in the helicopter transmission system.