Allergic rhinitis and asthma are common diseases that frequently occur together. This concept has been referred to in the literature as united airway disease. Epidemiological studies have shown that the majority of patients with asthma have concomitant rhinitis and the presence of rhinitis is an increased risk factor for development of asthma. Patients with asthma and rhinitis share common physiology including heightened bronchial hyperresponsiveness and heightened reactivity to a variety of stimuli. Immunopathology of allergic rhinitis is also similar with the predominance of T-helper type 2 inflammation and tissue eosinophilia. Although several mechanisms have been proposed to explain the united airway theory, some of the best lines of evidence suggest that local airway inflammation can result in a systemic inflammatory response. Pharmacotherapeutic studies have shown that the treatment of rhinitis can improve asthma and vice versa. Nevertheless, systemic approaches such as immunotherapy lead to better outcomes for treating both disease states simultaneously. This article will focus on the data supporting the common epidemiology, shared pathophysiology, and therapeutic interventions aimed at allergic rhinitis and asthma as united airway diseases.