Although the upper and lower airways have distinct functions and are managed by different specialists, they also share many characteristics. Problems of the nose and sinuses can impact on the lungs, and vice versa. This ERS Monograph presents an up-to-date overview of the subject area, starting with chapters covering basic anatomy, imaging and practical clinical examination. Other chapters focus on associations with cystic fibrosis, infections, asthma, COPD, non-cystic fibrosis bronchiectasis, primary ciliary dyskinesia and air pollution. The united airways are discussed in terms of the impact of optimal upper airways treatment on lower airways disorders and current guidelines to aid all physicians, especially ENT and chest specialists, in clinical practice.