Exercise limitation is a common and disturbing manifestation of COPD. The exercise intolerance is often caused by multiple interrelated anatomic and physiologic disturbances. Importantly, exercise tolerance can be improved despite the presence of fixed structural abnormalities in the lung. Exercise training, undertaken alone or in the context of comprehensive PR, improves exercise endurance and, to a lesser degree, the maximal tolerated workload of patients with COPD. Pulmonary rehabilitation also improves dyspnea and QOL. Exercise training and PR should be considered for all patients lacking contraindications who experience exercise intolerance despite optimal medical therapy. Lower-extremity training should be included routinely in the exercise prescription. The choice of type and intensity of training should be based primarily on the patient's individual baseline functional status, symptoms, needs, and long-term goals. When tolerated, high-intensity (continuous or interval) training may lead to greater improvements in aerobic fitness than low-intensity training but is not absolutely necessary to achieve gains in exercise endurance. Upper-extremity training should be undertaken when possible. Ventilatory muscle training should be considered for patients who continue to experience exercise limitation and breathlessness despite medical therapy and general exercise reconditioning. Exercise tolerance may improve following exercise training because of gains in aerobic fitness or peripheral muscle strength; enhanced mechanical skill and efficiency of exercise; improvements in respiratory muscle function, breathing pattern, or lung hyperinflation; as well as reduction in anxiety, fear, and dyspnea associated with exercise. Gains made in exercise tolerance can last up to 2 years following a limited duration (6-12 week) rehabilitation program.