Exercise testing is a valuable tool to assess cardiopulmonary function. It can define the limits of performance in an athlete or the functional capabilities of the patient with heart disease. Exercise capacity is determined most accurately by ventilatory oxygen uptake, which is defined by maximal cardiac output and the ability to extract and utilize oxygen in the working muscle. A given increase in cardiac output must be matched by a concomitant increase in ventilation, such that efficient pulmonary gas exchange can occur. The increase in cardiac output required by an exercise test places major demands on the heart muscle; the increase in myocardial oxygen demand can bring out abnormalities not apparent at rest. Thus, the exercise test is useful clinically to assess cardiac perfusion and function under controlled conditions. The physiologic response to exercise is influenced by the state of health and fitness, age, gender, the type of exercise, exercise position, and the environment. A good understanding of the basic physiologic responses to acute exercise can assist the clinician in applying the information gained from the exercise test to patients with cardiovascular disease. In addition to diagnostic and prognostic information, these applications include the assessment of therapy, exercise prescription, and hemodynamic responses.