A first aspect of the invention provides a method of testing a yaw system (200) of a wind turbine, the wind turbine comprising a rotor; the yaw system (200) comprising a yaw gear (202) coupled to the rotor so that rotation of the yaw gear (202) causes yaw rotation of the rotor, and first and second sub-systems (204a, 204b), the first sub-system (204a) comprising a first pinion gear (206a) and a first drive motor (208a) coupled to the yaw gear (202) by the first pinion gear (206a), the second sub-system (204b) comprising a second pinion gear (206b) and a second drive motor (208b) coupled to the yaw gear (202) by the second pinion gear (206b). The method comprises the steps of: testing the first sub-system (204a) by: applying a first yaw moment to the yaw gear (202) with the second drive motor (208b) via the second pinion gear (206b), reacting the first yaw moment with the first pinion gear (206a), monitoring a yaw motion parameter indicative of rotation of the yaw gear (202), and determining a condition of the first sub-system (204a) based on the monitored yaw motion parameter.