A wind turbine blade is described wherein at least on planar member is provided on the blade surface, where the planar member is arranged such that it extends at an angle to the chord of the blade. The planar member acts to re-direct airflow over the blade, to improve wind turbine performance. The planar member may be a stall fence provided towards the blade root end, further acting to divert airflow towards the root end of the blade to prevent separation of attached airflow. Additionally or alternatively, the planar member may be a flow diverter provided towards the blade tip end, to increase airflow in the tip region for increased performance and/or to disrupt the formation of tip vortices.