An instrument for positioning a cup component of an orthopaedic joint prosthesis includes a shaft, and a flange located near an end of the shaft on which the cup component can be fixed. The flange extends from the shaft transversely, and can be retracted from an in-use position wherein the flange is received in a groove in the internal surface of the cup component when positioned over the said end of the instrument, and a retracted position in which the flange is withdrawn toward the axis of the shaft from the in-use position, allowing the cup component to be released from the instrument.