Respiratory muscle weakness is observed in chronic obstructive pulmonary disease (COPD) patients and contributes to hypoxemia, hypercapnia, dyspnoea, nocturnal oxygen desaturation and reduced exercise performance. During exercise it has been shown that diaphragm work is increased in COPD and COPD patients use a larger proportion of the maximal inspiratory pressure (PImax) than healthy subjects. This pattern of breathing is closely related to the dyspnoea sensation during exercise and might potentially induce respiratory muscle fatigue. However, diaphragmatic fatigue was not demonstrated after exhaustive exercise. Studies in patients with COPD have shown natural adaptations of the diaphragm to greater oxidative capacity and resistance to fatigue. The above mentioned considerations gave conflicting arguments to the rationale of inspiratory muscle training (IMT) in COPD. Both IMT alone and IMT as adjunct to general exercise reconditioning significantly increased inspiratory muscle strength and endurance. A significant effect was found for dyspnoea at rest and during exercise. Improvedfunctional exercise capacity tended to be an additional effect of IMT alone and as an adjunct to general exercise reconditioning, but this trend did not reach statistical significance. No significant correlations were found for training effects with patient characteristics. However, subgroup analysis in IMTplus exercise training revealed that patients with inspiratory muscle weakness improved significantly more compared to patients without inspiratory muscle weakness. From this review it is concluded that inspiratory muscle training is an important addition to a pulmonary rehabilitation programme directed at chronic obstructive pulmonary disease patients with inspiratory muscle weakness. The effect on exercise performance is still to be determined. In summary, IMT improves inspiratory muscle strength and endurance, functional exercise capacity, dyspnoea and quality of life. Inspiratory muscle endurance training was shown to be less effective than respiratory muscle strength training.