A diagnostic system for use in a wind turbine yaw system, comprising: a tower motion sensor configured to output a signal indicative of tower oscillation, in particular though not exclusively side to side tower oscillation, and a diagnostic module configured to: analyse the tower motion sensor signal to identify frequency content of the signal that is not associated with the tower oscillation; and correlate the identified frequency content with the operation of the yaw system thereby to determine that the yaw system requires maintenance. Beneficially the invention provides that the health of the yaw system can be determined by analysing the oscillatory movement of the tower as measured by a tower motion sensor installed at a suitable location for example at the top of the tower or in the nacelle for example.