Objective: We report on two years of clinical experience with a frameless Computer Aided Surgery system developed in Bern, Switzerland. Material and Methods: Our navigation system is based on a preoperative computer-tomography (CT) scan (without markers) and an intraoperative optical tracking of head movements and of the surgical instruments. Using landmark and surface-based registration, the skull can be accurately correlated to the CT images. The three-dimensional positions of the surgical instruments, as well as the endoscopic images, are displayed in real time on a monitor. Results: In the last two years, 109 computer-aided interventions have been successfully performed: 89 on the anterior skull base/paranasal sinuses, 15 on the lateral skull base, and 5 minimally invasive procedures on other locations in the skull. No complications occurred. The practical accuracy on the cadaver skull is between 0.5 mm and 1.2 mm, and the clinical accuracy is between 0.5 mm and 2 mm. Conclusions: Our navigation system has proven its accuracy and usability. Surgeons feel very comfortable with the increased safety provided by the unequivocal identification of important anatomical structures.