© Cambridge University Press (2008) 2015. Introduction Aspiration is the introduction of oropharyngeal or gastric contents into the respiratory tract. Three major syndromes may develop as a consequence of aspiration: chemical pneumonitis, bronchial obstruction secondary to aspiration of particulate matter, and bacterial aspiration pneumonia. Less commonly, interstitial lung disease occurs in persons with chronic aspiration. Which of these consequences emerges is determined by the amount and nature of the aspirated material as well as by the integrity of host defense mechanisms. The term aspiration pneumonia refers to the infectious consequences of introduction of relatively large volumes of oral material into the lower airways (macroaspiration). Although healthy persons frequently aspirate small volumes of pharyngeal secretions during sleep, the development of pneumonia after such microaspiration is normally prevented by mechanical (e.g., cough and mucociliary transport) and immunologic responses. Pneumonia arises when these host defenses are not able to limit bacterial proliferation either because of microaspiration of highly virulent pathogens to which the host lacks specific immunity (e.g., Streptococcus pneumoniae or enteric gram-negative bacteria) or because of macroaspiration of large quantities of organisms that may not necessarily be highly virulent. Aspiration may be clinically obvious, as when acute pulmonary complications follow inhalation of vomited gastric contents. Such acute chemical pneumonitis, representing damage to lung parenchyma by highly acidic gastric contents, is often referred to as Mendelson’s syndrome. On the other extreme, so-called silent aspiration, as occurs in persons with neurologic impairment who lack cough responses, is often followed by the indolent onset of infectious pneumonia consequent to contamination of the lower airways by low virulence mixtures of aerobic and anaerobic microorganisms from the oropharynx.