Chronic obstructive pulmonary disease, COPD is a highly prevalent disorder of increasing medical and socio-economical importance. It is characterized by irreversible airflow obstruction. Besides airflow obstruction also other features are present. One of these is respiratory muscle weakness. Inspiratory muscle weakness is caused by hyperinflation and by generalized muscle weakness causing both respiratory and peripheral muscle dysfunction. The expiratory muscles partake in this generalized muscle weakness. Hyperinflation shortens the inspiratory muscles although in chronic hyperinflation sarcomere adaptation occurs. Generalized muscle weakness is caused by deconditioning, malnutrition, electrolyte disturbances, cardiac failure, systemic inflammation and treatment with corticosteroids causing steroid-induced myopathy. The latter disease was studied intensively both in patients and in animal models of disease. The major findings were that microscopically a myopathic pattern was present associated with generalized fiber atrophy. This is in contrast to classical belief that the atrophy would be confined to type IIx fibers. We noted severe down-regulation of the IGF-I mRNA expression, without important changes in the expression of the binding proteins. This may be responsible for the observed muscle atrophy and the myopathy. The latter is likely to be caused by a simultaneous upregulation of the ubiquitin protease pathway attacking structural proteins. Presently, we study the relationship between local and systemic cytokine expression and respiratory and peripheral muscle dysfunction in COPD patients. Respiratory and peripheral muscle dysfunction have significant consequences for COPD patients. Both respiratory and peripheral muscle dysfunction are associated with reduced exercise tolerance and reduced quality of life. Both are independent determinants of survival, in addition to the degree of airflow obstruction as measured by FEV1. Finally, also the utilization of health care resources appeared to be related to respiratory and peripheral muscle weakness. Treatment of respiratory and peripheral muscle weakness in COPD patients is possible. Respiratory and peripheral muscle training have been shown to produce beneficial effects. Nutritional intervention and anabolic steroids are only useful in combination with muscle training. Systemic administration of growth hormone and IGF-I only produces small effects. In animal models, local administration of IGF-I and transfer of the IGF-I gene transfer appear more promising for the future. Lung volume reduction surgery, LVRS, improves the force-generating capacity of the inspiratory muscles, presumably because of the geometrical alterations it causes in these muscles. It does not appear to improve intrinsic inspiratory muscle function.