Respiratory tract secretions consist of mucus, surfactant, and periciliary fluid. The airway surface fluid is present as a bilayer, with a superficial gel or mucous layer and a layer of periciliary fluid interposed between the mucous layer and the epithelium. A thin layer of surfactant separates the mucous and periciliary fluid layers. The mucous layer extends from the intermediate airway to the upper airway and is approximately 2-10 microm thick in the trachea. Airway mucus is the secretory product of the goblet cells and the submucosal glands. It is a nonhomogeneous, adhesive, viscoelastic gel composed of water, carbohydrates, proteins, and lipids. In health, the mucous gel is primarily composed of a 3-dimensional tangled polymer network of mucous glycoproteins or mucin. Mucin macromolecules are 70-80% carbohydrate, 20% protein, and 1-2% sulfate bound to oligosaccharide side chains. The protein backbones of mucins are encoded by mucin genes (MUC genes), at least 8 of which are expressed in the respiratory tract, although MUC5AC and MUC5B are the 2 principal gel-forming mucins secreted in the airway. Mucus is transported from the lower respiratory tract into the pharynx by air flow and mucociliary clearance. Expectorated sputum is composed of lower respiratory tract secretions along with nasopharyngeal and oropharyngeal secretions, cellular debris, and microorganisms. Disruption of normal secretion or mucociliary clearance impairs pulmonary function and lung defense and increases risk of infection. When there is extensive ciliary damage and mucus hypersecretion, airflow-dependent mucus clearance such as cough becomes critically important for airway hygiene.