A method for controlling a multi-rotor wind turbine generator having at least two rotor nacelle assemblies mounted to a support arrangement by a common yaw control system and each having a wind direction sensor mounted thereto configured to measure a wind direction relative to forward direction of its rotor nacelle assembly , wherein the method comprises: measuring , for each rotor nacelle assembly , wind power parameter data for the rotor nacelle assembly over a plurality of wind directions measured by the wind direction sensor ; either, in a first alternative: determining , for each rotor nacelle assembly , the wind direction at which the wind power parameter has a maximum value; determining a control wind direction as the average wind direction of the determined wind directions at which the wind power parameter has its maximum value; or, in a second alternative: combining the wind power parameter data from each rotor nacelle assembly to provide an overall wind power parameter data set for the rotor nacelle assemblies over a plurality of common wind directions; determining , from the overall measurement set, a control wind direction as the wind direction at which the wind power parameter has a maximum value; controlling the common yaw control system based on the determined control wind direction; characterized in that the method further comprising recalibrating each wind direction sensor to set the wind direction at which the wind direction sensor measures zero degrees as equal to the control wind direction or to set the wind direction at which the wind direction sensor measures zero degrees as between the control wind direction and the current wind direction at which the sensor measures zero degrees. measuring , for each rotor nacelle assembly , wind power parameter data for the rotor nacelle assembly over a plurality of wind directions measured by the wind direction sensor ; either, in a first alternative: determining , for each rotor nacelle assembly , the wind direction at which the wind power parameter has a maximum value; determining a control wind direction as the average wind direction of the determined wind directions at which the wind power parameter has its maximum value; determining , for each rotor nacelle assembly , the wind direction at which the wind power parameter has a maximum value; determining a control wind direction as the average wind direction of the determined wind directions at which the wind power parameter has its maximum value; or, in a second alternative: combining the wind power parameter data from each rotor nacelle assembly to provide an overall wind power parameter data set for the rotor nacelle assemblies over a plurality of common wind directions; determining , from the overall measurement set, a control wind direction as the wind direction at which the wind power parameter has a maximum value; combining the wind power parameter data from each rotor nacelle assembly to provide an overall wind power parameter data set for the rotor nacelle assemblies over a plurality of common wind directions; determining , from the overall measurement set, a control wind direction as the wind direction at which the wind power parameter has a maximum value; controlling the common yaw control system based on the determined control wind direction; <EOS>