The upper respiratory tract lies in continuity with the lower respiratory tract. Both are lined by an epithelium that is broadly similar, and many of the same physical principles govern the flow of air through both. Several diseases such as allergic rhinitis and asthma frequently coexist, leading to the term ‘united airway disease’. The diagnostic approach to the lung and sinus disease is bidirectional. On one hand, certain lung diseases must particularly be looked for when sinusitis is severe. On the other hand, the presence of sinus disease should routinely be looked for when a diagnosis of certain lung diseases is made. This chapter discusses the purported functions of the paranasal sinuses in health, and outlines the clinical and imageological manifestations of chronic sinusitis. It explores the relationship between chronic sinusitis and bronchopulmonary disease and discusses the implication of specific lung diseases when they exist in conjunction with chronic sinusitis.