Dyspnea may be defined as an uncomfortable sensation of breathing. The sense of respiratory effort, chemoreceptor stimulation, mechanical stimuli arising in lung and chest wall receptors, and neuroventilatory dissociation may all contribute to the sensation of dyspnea. Different mechanisms likely give rise to qualitatively different sensations of dyspnea. In most patients, dyspnea is probably due to a combination of mechanisms. For example, in asthma, a heightened sense of effort, neuroventilatory dissociation, and vagal stimuli arising from bronchoconstriction and airway inflammation may all play a role. Patients with different disorders and different mechanisms of dyspnea use different phrases to describe their breathing discomfort. Hence, the language patients use to describe their dyspnea may provide clues to the etiology of their symptoms.