Patients with chronic obstructive pulmonary disease (COPD) describe their breathlessness as related to the work and effort associated with breathing. Current evidence suggests that the perception of dyspnea is due to a "mismatch" between the outgoing motor command from the central nervous system and the corresponding afferent information from chemoreceptors and/or mechanoreceptors. To measure the severity of dyspnea the principles of psychophysics (stimulus --> response relationship) can be applied. One approach is to consider activities of daily living as a putative stimulus. Although this method relies on patient recall and description of daily tasks, ability to function, as well as time and effort to complete an activity, select clinical instruments have demonstrated appropriate measurement criteria in randomized clinical trials involving patients with COPD. Another approach is for a patient to report the intensity of dyspnea during exercise, and current practice is to provide ratings each minute "on cue" during the exercise test. A computerized system has been developed whereby the person can report ratings spontaneously and continuously by moving a computer mouse that adjusts a vertical bar adjacent to 0-10 category-ratio scale positioned on a monitor. With this continuous method the patient reports twice the number of dyspnea ratings during exercise compared with discrete ratings each minute. Patient-reported dyspnea based on activities of daily living and exercise testing provides distinct but complimentary information.