A method is disclosed for controlling a wind turbine generator to provide power above a rated level based upon operational constraints. The wind turbine includes an over-rating controller that calculates an over-rating power demand in response to the values of one or more operating parameters, and communicates this demand to the generator. An over-rating command value is calculated according to set points of the operating parameter(s), the calculation taking into account the extent to which the operating parameter differs from these set points. The over-rating command value may vary proportionately with the difference between the operating parameter and the associated set point. If multiple operating parameters are used then the over-rating power demand may correspond to the minimum over-rating command value. Furthermore, the power demand maybe communicated to wind turbine generators individually, or may be calculated for a wind turbine park.