The measurement of dyspnea during an exercise task provides an opportunity to simulate daily physical activities that lead to breathing difficulty in patients with lung disease. Although the exact stimulus for the sensation of breathlessness is unknown, it is possible to measure dyspnea during exercise by applying the principles of psychophysics to the analysis of various stimulus-response relationships. It is logical to consider that the exercise task, ie, work or power production, causes both physiologic and perceptual responses. A 0 to 10 category scale with ratio properties developed by Borg and a visual analogue scale are the most commonly used instruments for rating the severity of dyspnea during exercise. The ratings of breathlessness are generally reliable over time and are sensitive to evaluate an acute intervention in patients with stable respiratory disease. The exercise intensity-dyspnea relationship appears to be the most appropriate stimulus-response relationship for quantifying dyspnea during exercise.