The intricate anatomy of the paranasal sinus unit (sinus/nasal cavities and ostia) and its relations to the orbit and anterior cranial fossa make accurate pre-operative evaluation of disease in this region mandatory before undertaking functional endoscopic sinus surgery (FESS). Plain radiography often fails to provide vital information in cases of chronic rhinosinusitis, especially of the ethmoid labyrinth, which is a key area of the paranasal sinus unit. Computerized tomography has significantly enhanced the scope of endoscopic sinus surgery by offering the surgeon hitherto inaccessible information regarding the sinus status. In this article, we review the pros and cons of the two imaging modalities, and present our experience fo 70 CT scans in patients of FESS. We also propose a surgical classification of CT findings. All in all, plain radiographs prove to be of little value for pre-operative assessment in FESS, while CT scans are an essential tool.