The airways are cleared by mucociliary action and coughing. Mucociliary clearance depends on normal active beating cilia coupled to a mucous layer, the physical properties of which permit efficient cephalic movement. Optimal viscosity and elasticity are important. Coughing, essentially a reserve mechanism most efficient in the central airways, relies on a high linear airflow velocity generated by ample flow, and airway narrowing with a two-phase, air-liquid flow regime. This regime may be curtailed because of inadequate flow in patients with chronic obstructive pulmonary disease (COPD). Failure of these two mechanisms may lead to sputum retention in the post-operative state or acute-on-chronic bronchitis, with respiratory failure. In e.g. primary ciliary dyskinesia, as in Kartagener's syndrome (sinusitis, bronchiectasis and situs inversus) and Young's syndrome (primary male infertility), the cardinal features are related to mucociliary insufficiency. Similarly, in asthmatics sputum plugging may occur, even in mild asthma. Clearance may be improved by drugs such as beta 2-agonists, theophylline, corticosteroids and mucolytics in various disease states. Physical means, such as gravitational postural drainage, and the forced expiration technique (FET) used for chest physiotherapy, may also be effective.