Chronic rhinosinusitis can be differentiated into several phenotypes based on clinical criteria; however, these phenotypes do not teach us much about the underlying inflammatory mechanisms. Thus, the use of nasal endoscopy and CT scanning, and eventually taking a swab or a biopsy, may not be sufficient to fully appreciate the individual patient's pathology. Endotyping of chronic rhinosinusitis on the basis of pathomechanisms, functionally and pathologically different from others by the involvement of specific molecules or cells, may in contrast provide us with information on the risk of disease progression or recurrence and on the best available treatment, and also helps us identifying innovative therapeutic targets for treatment. Endotyping may best be structured around T helper cells and their downstream events, such as tissue eosinophilia or neutrophilia; this approach involves the cytokines and chemokines related to specific T helper cell populations, and related markers such as IgE. Endotyping is of specific interest at the time of the arrival of new biologicals, confronting us with the challenge of the selection of eligible patients for treatment and predicting their therapeutic response; defining suitable biomarkers is therefore an urgent task. Failure to appreciate the underlying mechanisms and endotypes of chronic rhinosinusitis may limit progress in the management of the disease at present.