Dyspnea, the sensation of feeling breathless, is a symptom experienced under conditions in which there is an inordinately high ventilatory demand relative to the ability to breathe. Its major physical sign is tachypnea. New developments in monitoring ventilation during exercise have improved our ability to evaluate the symptom of dyspnea and to understand pathophysiological mechanisms contributing to the symptom. We briefly describe the range of mechanisms that determine exercise ventilation and their possible relationship to dyspnea. Questionnaires and psychophysical testing have been used to quantify dyspnea, but there is variability in dyspnea grade from these methods. Dyspnea-producing stimuli and the mechanisms by which they act are reviewed. Disorders producing dyspnea and the pathophysiological mechanisms underlying each are discussed. Perception of dyspnea is obviously through the central nervous system, where dyspnea-producing stimuli are integrated. The specific integration site is probably in the region of the brain stem, since occasional patients with brain stem lesions do not experience dyspnea despite the presence of a number of dyspnea-producing stimuli.