A surgical instrument system includes a tool (2) including an elongate shaft which defines the tool axis. The shaft bears a plurality of marker rings (10, 11, 12) arranged in a predetermined pattern on the surface of the shaft so that they extend around the shaft axis. The system includes at least two receiving devices (14) which are spaced apart for receiving stereoscopic signals from the rings on the tool, and a data processor (16) for analyzing the signal from the rings and generating information relating to the position and orientation of the tool relative to the receiving device.
