An adjustable shim comprises a stack of plies comprising plastics material reinforced with a three-dimensional fiber layer, the plastics material being flowable under compressive force prior to cure for adjusting the thickness of the shim. The shim may be used to fill a void between two structural components joined together. Also, a method of filling a void between two structural components using the shim. The shim is easy to handle, can easily be laid up to a variety of thicknesses, and is quick to cure (depending on the plastics material used). The three-dimensional fiber layers make the shim highly compressible prior to cure, and provide significant reinforcement to the plastics material of the plies to support compressive stresses in the shim post cure.