Dyspnea and leg effort are the major symptoms limiting exercise in healthy subjects and in patients with a variety of respiratory disorders. Quantitative measurement of both symptoms may be obtained by category scales such as VAS and Borg, with the latter being widely used. Furthermore, descriptor clusters of dyspnea help to assess some of the reasons for stopping exercise. The intensity of dyspnea and leg effort are similar in different disease states; this symmetry suggests that the limiting discomfort is a function of the intensity of increased motor drive to peripheral and respiratory muscles. An alternative explanation for the factors which limit exercise is that the subjects stop exercise volitionally when the discomfort associated with continuing exercise exceeds that which they are willing to tolerate. Muscle strength contributes to the intensity of dyspnea and leg effort at a given power output: the greater the muscle force, the lower the symptom. Symptoms also correlate with intensity and duration of a task by a power function in such a way that when minimizing the intensity of a given muscular task by prolonging the duration of activity, the symptom is drastically reduced. Skeletal muscle fatigue may be a factor limiting exercise tolerance both in healthy subjects and in patients with cardiorespiratory disorders. In conclusion, symptom measurement complements physiological measurements, both being essential to a comprehensive understanding of exercise tolerance.