Dyspnea is a cardinal symptom of asthma and may arise from several pathophysiological mechanisms, including pulmonary hyperinflation, stimulation of vagal receptors, and, rarely, chemoreceptor stimulation. The language that patients use to describe their breathlessness may provide important clues about the physiology underlying symptoms in a particular patient. Several physiological derangements may contribute to dyspnea in a given individual. The variability in the severity of breathlessness for any given degree of airflow obstruction may relate to differences in the relative importance of these physiological changes and/or to a range of perceptual abilities in asthmatic patients. One hypothesis that is under current investigation is that defective perception of asthma symptoms may lead to undertreatment and the potential for greater morbidity and mortality from asthma.