Acquired myopathies are a group of diseases in which the primary pathology causing muscle weakness and wasting lies in the muscles. They can be divided into three major groups: idiopathic inflammatory myopathies (IIMs) and infectious and nonimmune myopathies (metabolic, endocrine, and toxic). The prevalence of IIMs is low, whereas nonimmune myopathies are frequent. Etiological agents for IIMs are unknown, but genetic factors may have a role. A number of causes for toxic, endocrine, and metabolic myopathies have been reported. IIMs, a heterogeneous group of autoimmune rheumatic diseases, are defined by criteria that include clinical features, serological abnormalities, electromyographic changes, and typical histological appearance. Exact diagnostic criteria remain undefined because various proposed criteria have not been properly validated. It is generally believed that the diagnosis should rely on histopathology and immunopathology. In polymyositis and inclusion body myositis, class I MHC antigens expressed on all muscle fibers seem to be the target of CD8 + autoinvasive T cells. In contrast, the main effector response in dermatomyositis is humoral, directed against endothelial cells of the microvasculature. Corticosteroids alone or in combination with azathioprine, methotrexate, cyclosporine, cyclophosphamide, and mycophenolate are the therapeutic agents of choice. Adequate double-blind controlled clinical trials are missing, but it seems certain that corticosteroids have substantially improved the outcome of patients with IIMs.