A method of monitoring the condition of a yaw system of a wind turbine , the wind turbine comprising a rotor, the yaw system arranged to control a yaw rotation of the rotor, the method comprising: providing design data representing an expected relationship between yaw moment and yaw rotation speed represented in a region of a parameter space, said design data comprising a design curve and at least one of an upper boundary curve above said design curve and a lower boundary curve below said design curve ; - said upper boundary curve dividing said parameter space into a region above said upper boundary curve , said region comprising a loose and a stuck region, and a normal region between said upper boundary curve and said design curve ; - said lower boundary curve dividing said parameter space into a normal region between said lower boundary curve and said design curve and region below said lower boundary curve , said region below said lower boundary curve comprising a stuck and a loose region; measuring a pair of parameters, the pair of parameters comprising a yaw moment parameter indicative of a yaw moment applied to the yaw system, and a yaw rotation speed parameter indicative of a yaw rotation speed caused by the yaw moment; using the design data to evaluate whether the pair of parameters deviates from the expected relationship; and determining a condition of the yaw system on the basis of the evaluation and whether the pair of parameters falls inside or outside the normal region of the parameter space. providing design data representing an expected relationship between yaw moment and yaw rotation speed represented in a region of a parameter space, said design data comprising a design curve and at least one of an upper boundary curve above said design curve and a lower boundary curve below said design curve ; - said upper boundary curve dividing said parameter space into a region above said upper boundary curve , said region comprising a loose and a stuck region, and a normal region between said upper boundary curve and said design curve ; - said lower boundary curve dividing said parameter space into a normal region between said lower boundary curve and said design curve and region below said lower boundary curve , said region below said lower boundary curve comprising a stuck and a loose region; - said upper boundary curve dividing said parameter space into a region above said upper boundary curve , said region comprising a loose and a stuck region, and a normal region between said upper boundary curve and said design curve ; - said lower boundary curve dividing said parameter space into a normal region between said lower boundary curve and said design curve and region below said lower boundary curve , said region below said lower boundary curve comprising a stuck and a loose region; measuring a pair of parameters, the pair of parameters comprising a yaw moment parameter indicative of a yaw moment applied to the yaw system, and a yaw rotation speed parameter indicative of a yaw rotation speed caused by the yaw moment; using the design data to evaluate whether the pair of parameters deviates from the expected relationship; and determining a condition of the yaw system on the basis of the evaluation and whether the pair of parameters falls inside or outside the normal region of the parameter space. <EOS>