Dyspnea is the medical term for the patient's or subject's complaint of shortness of breath. It encompasses the respiratory discomfort experienced in many different diease states as well as the shortness of breath felt by a normal subject during or after strenuous exercise. Several parameters which have been shown to correlate with the onset or severity of dyspnea are described, including reduced vital capacity, the ratio of minute ventilation to vital capacity, reduced breathing reserve, the work of breathing, and the oxygen cost of breathing. Attempts at quantitation of dyspnea have usually consisted of measuring physiological parameters associated with the sensation, such as the "dyspneic index". The direct measurement of respiratory sensations using modern psycho-physical methods is at an early stage of development. Since the observation that the existence of dyspnea is often unrelated to any disturbance of arterial blood gas composition, it has been generally held that the mechanism of dyspnea is primarily neurophysiological. The neural pathways may conceptually be divided into those which transmit the "dyspnea message" from the respiratory apparatus to integrating centers in the brain, and those concerned with subsequently bringing the sensation to the level of consciousness. It seems likely that there is no single sensing mechanism and neural pathway which will be able to explain dyspnea in the diverse populations of patients and subjects who experience unpleasant respiratory sensations. Three theories concerning mechanisms of dyspnea are briefly described: "length-tension inappropriateness", vagal afferent activity especially from the J-receptors, and the recent concept of diaphragmatic fatigue. Some specific characteristics of the shortness of breath experienced in certain disease states are described, including chronic bronchitis and emphysema, bronchial asthma, pulmonary fibrosis and congestive heart disease.