Abstract: The apparatus comprises video cameras for viewing part of a patient's body and a slide projector for projecting a predetermined pattern of light onto the patient's body which is viewed by the cameras. A workstation and video frame grabber performs controlling and processing functions and are arranged to process said images from the cameras using image processing algorithms. The images from the cameras are superimposed upon prestored images which are generated from X-ray or CT scans. A passive pointer, having a predetermined pattern thereon which is recognisable by the controlling and processing means is used by the surgeon. The pattern is used to identify the exact position of a tip of the pointer in relation to the patient's body and is displayed on a monitor with the superimposed images.