This chapter discusses classification and diagnosis of rhinosinusitis (RS), which is a complex group of disorders characterized by infection and inflammation of the mucosa of the nose and paranasal sinuses. RS has been characterized by duration of symptoms and objective findings into acute, recurrent acute, subacute, and chronic RS (CRS). The viruses alone can cause inflammation in the sinuses, and this has been confirmed by computed tomographic scans performed during a viral upper respiratory tract infection. Chronic rhinosinusitis is a group of disorders characterized by inflammation of the mucosa of the nose and paranasal sinuses of at least twelve consecutive weeks duration. Common causes of chronic sinusitis include viral, bacterial, fungal, allergic and idiopathic. CT scan of the sinuses without intravenous contrast is considered the gold standard for radiographic evaluation of the paranasal sinuses. Nasal and paranasal sinus cultures have played an important role in treating RS.