Exercise intolerance in COPD patients appears to be in part because of skeletal muscle dysfunction. Studies using biopsy techniques and magnetic resonance spectroscopy have demonstrated changes in enzyme activities and metabolism that indicate reduced oxidative capacity in the peripheral muscles of these patients. Regarding the respiratory muscles, the biochemical characteristics have been studied in only a few works and the results seem to depend on the specific muscle group studied. Several factors, such as hypoxemia, nutritional status, pharmacological treatment, and deconditioning, may be responsible for these skeletal muscle abnormalities. This brief review describes the changes in peripheral and respiratory muscles in COPD patients based on data from the literature. The causes of these muscle abnormalities, their contribution to exercise intolerance, and the effects of training are then discussed. We conclude with suggested directions for future investigation using contemporary noninvasive technologies.