In clinical practice the nose and the lungs are considered separately and are attended by different specialists. However, nasal and bronchial symptoms and diseases often coexist, as is commonly the case with rhinitis and asthma [1–3], The upper and lower respiratory airways form a continuous tract, share many anatomical and histological properties and one important function, the passage of air in and out of the lungs. Furthermore, the upper and lower airways share a common susceptibility to various agents such as allergens, infectious agents, occupational sensitizers and drugs, and respond to these in a similar fashion [4–11]. Nevertheless, each part of the airway has specialized functions: humidification, filtration of the air and the sense of smell in the nose, phonation in the larynx, and gas exchange in the lungs. In this chapter, the anatomy, histology, physiology and function of the respiratory tract is briefly described, the triggering mechanisms producing physiological and immunological responses and symptoms are discussed and the differences and simi-larities between the upper and lower airways are pointed out. Anatomy The respiratory tract is functionally divided into the conducting and the gas exchanging part. The conducting airways are anatomically subdivided into the nose and mouth, pharynx, larynx, trachea and bronchi, which keep branching down to the level of the respiratory bronchioles. The respiratory bronchioles, with the corresponding alveoli, represent the gas exchanging part of the lungs (fig. 1). Conventionally, the respiratory tract is divided into the upper and lower part and the larynx is the boundary between the two. The upper respiratory tract extends from the external nares and the mouth to the larynx and the lower from the larynx to the alveoli. The nasal cavity is surrounded by bone and divided into two halves by the nasal septum. The two separate airways extend from the anterior nares (nostrils) to the posterior ones, where they are united to form a single airway at the nasopharynx level. The nose has an external visible part surrounding the vestibule and an internal part containing the main nasal cavity. The nasal vestibules are trumpet-shaped orifices, narrowing towards the main nasal cavity. The junction between the vestibules and the main nasal cavity is the narrowest point of the airways and is called nasal valve or internal ostium. At this point the area of the airways is just 0.3 cm 2 expanding to nearly 1.3 cm 2 in the main cavity . In the main nasal cavity three bony shelves project from Eur Respir Mon, 2001, 18, 1–15. Printed in UK -all rights reserved. Copyright ERS Journals Ltd 2001; European Respiratory Monograph; ISSN 1025-448x. ISBN 1-904097-18-9.