This article reviews the classical basis on which dyspnea is identified and quantified. The sensation or sensations of discomfort experienced during breathing are then viewed using a background of sensory physiology. Exploiting psychophysical techniques, the origin of the sensation of discomfort is viewed within the constraints of the presently-known sensory structures. The contribution of these sensory structures to the quality and quantity of discomfort is suggested, indicating the central role played by the respiratory muscles.