Recent advances in "functional" endoscopic surgical techniques require the radiologist to understand the anatomy and pathophysiology of the nose and paranasal sinuses as well as their normal and abnormal CT patterns. Endoscopy and CT are complementary procedures, but in the evaluation of the extent of chronic inflammation and in the demonstration of possible associated local and regional complications, the latter method has become the imaging technique of choice, providing a detailed map for the functional sinus surgeon and replacing plain radiography. The authors reviewed a personal series of 322 coronal thin-slice CT examinations of the paranasal sinuses relative to 169 patients with chronic or acute sinusitis and polyposis. The main CT findings of sinusitis (mucosal thickening, fluid retention, mucous cysts, bone alterations, mucoceles and polyposis) and their complications are reported. Finally, ostiomeatal conditions are considered, particularly the obstruction of ducts and ostia which provide aeration and mucous clearance from the paranasal sinuses.