OBJECTIVE Most patients with significant chronic obstructive pulmonary disease (COPD) have inspiratory and expiratory muscle weakness. In addition, hyperinflation induces functional weakening of the inspiratory muscles, increased elastic load to breathing, and intrinsic positive end expiratory pressure (PEEPi). Therefore, it was rational to expect that patients with COPD would benefit from specific inspiratory or expiratory muscle training (SIMT, SEMT respectively). However, the functional benefits of SIMT have remained equivocal. In recent years, a number of studies have demonstrated that, when training loads are controlled, SIMT results in important functional benefits. The role of SEMT is still unclear. RESULTS Well-controlled SIMT in patients with COPD leads to relief of dyspnea, during both daily activities and during physical activity. This yields increased exercise tolerance, and thus the capacity to walk, improving health related quality of life. We argue that there is now evidence that SIMT is an important addition to pulmonary rehabilitation programs for patients with COPD. Although two recent studies have shown that SEMT also provides a beneficial effect in patients with COPD, this does not appear to be supplementary to the effect to SIMT. CONCLUSIONS Inspiratory and expiratory muscles can be specifically trained yielding improvements in both strength and endurance. The improvement in inspiratory muscle performance is associated with an improvement in the sensation of dyspnea, exercise tolerance, and quality of life. When the expiratory muscles are specifically trained, a significant increase in exercise performance has also been shown. However, there is probably no additional benefit in combining SEMT with SIMT.