Cough is a complex physiologic reflex which protects the lung from the inhalation of irritants and cleans the airways of excess secretions of solid particles. High linear velocities of flow in the airways are needed for the cough to be effective, and this depends upon the ability to achieve high expiratory flows and a reduction of airway caliber by dynamic compression. The efficacy of cough is compromised by a series of pathological conditions such as dysfunction of the cough reflex pathways, decrease in expiratory flows, abnormal dynamic compression, alterations in airway geometry, reduction in mucociliary clearance, and changes in bronchial secretions. Besides its protective action, cough may be responsible for adverse reactions or injuries.