A reinforcing structure for a wind turbine blade A reinforcing structure for a wind turbine blade (12) is described. The reinforcing structure comprises one or more pultruded strips (42C) having spanwise grooves (54). The grooves (54) impart transverse flexibility to the strips (42C), allowing the strips (42C) to conform to the curvature of a wind turbine blade mould (44). An associated method of making a reinforcing structure for a wind turbine blade (12) is described. The method comprises providing an elongate mould (44) extending in a longitudinal direction and defining a mould surface at least part of which is concave-curved in transverse cross section. One or more pultruded strips (42C) with spanwise grooves (54) are arranged in the mould (44) to form the reinforcing structure. The pultruded strip(s) are bent along the grooves (54) so that they substantially conform to the transverse curvature of the mould surface. In preferred embodiments the reinforcing structure is a spar cap (36).