The present disclosure relates to a system for determining at least one blade state parameter of a wind turbine blade, wherein the system is configured to: obtain blade data relating to the wind turbine blade from a sensor system associated with the wind turbine blade; compare at least one reference model of at least a portion of the wind turbine blade with the blade data; identify a reference model in dependence on the comparison; and determine at least one blade state parameter in dependence on the identified reference model. The blade data may take the form of an image, for example a 3-dimensional measurement such as a point cloud representing at least a portion of the blade.