A system for monitoring the deflection of a wind turbine blade is described. The system comprises a wireless range-measurement system, having at least one wireless communication device located towards the root end of the blade and at least one wireless communication device located towards the tip end of the blade. The root end device is provided on a bracket projecting from the external surface of the blade, to provide a communication path between the root end and tip end devices which is less susceptible to interference from multipath effects, etc. There is further provided a method to derive tilt and yaw moments from measured deflections. A control method for such a system is also described, wherein the signal gain of the communication path may be varied based at least in part upon the deflection characteristics of the wind turbine blade.