The lung is continually at risk of exposure to noxious environmental agents and respiratory pathogens. An elaborate series of defence mechanisms have been developed to protect the airways from these insults. The lower respiratory tract is protected by local mucociliary mechanisms that involve the integration of the ciliated epithelium, periciliary fluid and mucus. Mucus acts as a physical and chemical barrier onto which particles and organisms adhere. Cilia lining the respiratory tract propel the overlying mucus to the oropharynx where it is either swallowed or expectorated. Regulation of periciliary fluid is thought essential to maintenance of both mucociliary clearance and to produce an environment in which airway antimicrobial peptides and defensins are effective. Disruption of mucociliary clearance may be caused by diseases such as cystic fibrosis, primary ciliary dyskinesia and asthma or may be secondary to pollutant exposure and viral or bacterial infections.