The perception of breathlessness during physical activities is a frequent and disturbing complaint for patients with chronic respiratory disease. Psychophysical principles can be applied to quantitate the severity of dyspnea during cardiopulmonary exercise testing. Power production (or work) appears to be the most appropriate stimulus for measuring the dyspnea response. The 0-10 category-ratio (CR-10) scale and the visual analog scale (VAS) are two instruments for measuring the intensity of breathlessness. Studies demonstrate that the slope and/or intercept of the power production-breathlessness relationship provide a valid, reliable, and responsive approach for measurement. Ratings of dyspnea during exercise are useful to determine the severity of breathing difficulty and to assess the efficacy of therapy.