Polymyositis (PM), dermatomyositis (DM) and sporadic inclusion body myositis (s-IBM) are severe inflammatory muscle disorders of unknown cause, which may present life-threatening complications. Prognosis and response to medications may be predicted not only from the clinical and pathologic diagnostic group into which a patient belongs, but also from the patient's myositis-specific antibody status, extraskeletal muscle involvement, and the interval between onset of muscle weakness, and the start of the treatment. Corticosteroids remain the mainstay of treatment in PM and DM. In patients refractory or intolerant to corticosteroids, another therapy, often an immunosuppressive agent, or intravenous immunoglobulin (IVIg), is added. IVIg seems the treatment of choice in severe myositis with dysphagia. New molecules, anti-TNF and monoclonal antibodies anti-CD20 justifies randomised trial and long term follow up.