The term computer aided orthopedic surgery (CAOS) stands for approaches that aim to improve visibility to the surgical field and increase application accuracy by means of so-called navigation systems alone or in combination with smart end-effectors when carrying out surgical actions. These goals achieved by linking the bony anatomy being operated on with a virtual representation, such as an image dataset. This article introduces the basic principles of CAOS. Surgical navigation systems that use modern tracking technology are introduced and classified according to the chosen virtual representation of the surgical object, ie, image-free and image-based (preoperative and intraoperative) technology. Within the latter class in particular, CT-and fluoroscopy-based (2-D and 3-D) systems have successfully made their way into the operating room (OR). Challenges during the development of the underlying enabling technologies are presented and references to orthopedic applications in different anatomical areas are given.