A method of operating a wind turbine. The wind turbine is operated over an operating period in accordance with a control strategy. A sensor signal is received over the operating period from a sensor measuring an operational parameter of the turbine. A model is used to obtain a modelled fatigue value based on the sensor signal. The modelled fatigue value provides an estimate of fatigue loading applied to a component of the turbine over the operating period. The control strategy is modified based on the modelled fatigue value.