Abstract: To aid a medical practitioner in positioning a surgical instrument or implant when performing surgery on or examining portions of a patient, patient data is developed which identifies the position and orientation of the portion to which surgical procedures or examinations are to be applied. The position and orientation of the instrument or implant is sensed and instrument data is developed from this sensing. The patient data is converted to objective signals to be displayed on a video display, and the instrument data is converted to instrument signals for presenting the position and orientation of the instrument or implant on the same display. Thus, by watching the display, the medical practitioner is aided in manipulating the insrument or implant relative to the portion.