Allergic and nonallergic rhinitis affect approximately 30% of the U.S. population. Although allergic rhinitis has a clear definition and its pathophysiology has been thoroughly investigated, nonallergic rhinitis remains poorly defined and understood. There is consensus, however, that nonallergic rhinitis consists of a variety of heterogeneous conditions. In allergic rhinitis, the process of allergen sensitization involves the participation of antigen-presenting cells, T lymphocytes, and B lymphocytes and depends on environmental allergen exposure. Sensitization results in the generation of allergen-specific IgE that circulates in the peripheral blood and attaches itself on the surface of all mast cells and basophils including those that home to the nasal mucosa. Subsequent nasal exposure to allergen activates these cells and, through the release of the classic mediators of the allergic reaction, produces acute nasal symptoms. Over a short period of time, these symptoms become indolent, whereas inflammatory and immune cell infiltrate, including eosinophils, basophils, neutrophils, T lymphocytes, and monocytes, characterizes the late stages of the allergic response. In parallel, and probably as a result of the development of mucosal inflammation, the nose becomes primed to allergen and reacts more vigorously to subsequent allergen exposure but also becomes hyperresponsive to irritants and to changes in atmospheric conditions. In nonallergic rhinitis, several conditions may have been identified that are of interest for further research and phenotyping. These include a group of patients with apparent hyperresponsiveness of the C-fiber sensory nerves with no inflammatory changes in the nasal mucosa and a group with mucosal eosinophilia who may have allergic sensitization to common aeroallergens that is, however, manifested only in the nasal mucosa.