Many surgical procedures in the field of ENT take place in close proximity to vital structures like the orbit, the skull base, the internal carotid artery etc. In our clinic we have decided to study two different computer-assisted navigation systems to reduce the risk of trauma to these structures during endonasal endoscopic procedures. Such systems should be able to correlate the position of the surgical instrument, ideally in the submillimeter range, to CT- or MR-images. The ARTMA Virtual Patient finds the position of the instrument by permanently measuring magnetic fields. The ISG-system uses a mechanical arm to localize the probe in the patient. Using Computer-Assisted-Surgery (CAS) does not significantly extend the time needed for surgery. We found that it prevents the surgeon from inadvertently injuring structures. Other groups have shown (1) that CAS-systems minimize the risk of complications of surgical procedures in the frontobasis arising from mishaps in the usage of the endoscope or the surgical tool. If properly used, CAS-systems can be a very helpful tool in the hand of an experienced surgeon but will never replace his expertise and knowledge.