Monitoring and assessing power performance changes of one or more wind turbines of a wind farm. For each wind turbine to be monitored, a group of reference wind turbines is defined. During a training period a transfer function is generated for each monitored wind turbine. The transfer function establishes a relationship between locally measured wind speeds at each of the reference wind turbines and the power performance data for the monitored wind turbine obtained during the training period. During one or more subsequent test periods, measured power performance data for the monitored wind turbines is compared to predicted power performance data. The predicted power performance data is obtained by means of the locally measured wind speeds at the corresponding reference wind turbines during the test period(s) and the previously generated transfer function for the monitored wind turbine. This allows even small and/or gradual power performance degradation to be detected.