Although clinically evident aspiration is common in subjects with dysphagia, a significant proportion may aspirate silently, i.e., without any outward signs of swallowing difficulty. This article reviews the literature on the prevalence, etiology, and prognostic significance of silent aspiration. An electronic database search was performed using silent aspiration, aspiration, dysphagia, and stroke as search terms, together with hand-searching of articles. Silent aspiration has been described in many conditions and subgroups of patients (including normal individuals), using a number of detection methods, making comparisons a challenge. The best data are for acute stroke, in which 2%-25% of patients may aspirate silently. Mechanisms associated with silent aspiration may include central or local weakness/incoordination of the pharyngeal musculature, reduced laryngopharyngeal sensation, impaired ability to produce a reflexive cough, and low substance P or dopamine levels. In terms of prognosis, silent aspiration has been associated with increased morbidity and mortality in many but not all studies. However, some degree of silent aspiration at night may be normal in healthy individuals. The phenomenon of silent aspiration is poorly understood and further research is needed to improve methods of detection and thereby better define its prevalence and prognostic significance.