Tracking systems are critical to all types of computer-assisted image-guided intervention. Many technologies exist; however, only optical and electromagnetic systems are widely used commercially. In orthopaedic applications, optical systems dominate because of the large working volume and accuracy, but these systems suffer from deficiencies due to line of sight. Electromagnetic trackers can be made much smaller but are less accurate and are affected by metal, although current-generation systems are less affected by metal artifacts than were the earlier ones, which were more widely reported in the literature.