The upper respiratory tract (URT) is a morphologically complex organ system that plays a vital role in the filtration and humidification of inhaled air, in detecting environmental factors such as odors, and in mucosal immune defense against inhalant threats (Fig. 1.1). The efficiency of the URT in filtering airborne allergens is the reason allergic rhinitis (AR) is the most common atopic disorder. Extending from the nose to the larynx, the URT develops early in gestation, and its anatomy and function evolve through the teenage years. Each region of the URT has a distinct physiology influenced by various factors, including infections, irritants such as combustion products, humidity, medications, and chronic diseases. AR is a common condition affecting approximately 400 million people worldwide (Nur Husna et al. 2022). AR has been associated with chronic rhinosinusitis (CRS) and may exacerbate pre-existing CRS symptoms. Therefore, a subgroup of individuals with CRS has AR, although not every individual with AR will develop CRS. CRS with nasal polyps (CRSwNP) is a phenotypic subgroup of CRS typically mediated by type 2 inflammation (in Western countries), an inflammatory process shared with AR and characterized by eosinophilic mucosal infiltration. CRS tends to be associated with perennial rather than seasonal AR. In individuals requiring endoscopic surgery for medically refractory CRSwNP, more than half may have a perennial allergy (Helman et al. 2020). This chapter will describe the normal anatomy and physiology of the URT and the pathologic variations resulting from the immune dysfunction associated with AR and CRS.