The three major categories of idiopathic inflammatory myopathy are dermatomyositis (DM), polymyositis (PM), and inclusion body myositis (IBM). These disorders are clinically, histologically, and pathogenically distinct. DM is a humorally mediated microangiopathy, while PM is a T-cell-mediated disorder directed against muscle fibers. The pathogenesis of IBM is unknown. DM and PM are responsive to immunosuppressive therapy, in contrast to IBM, which is generally refractory to therapy. Prospective, double-blind, placebo-controlled trials are necessary to determine prognostic features and the best treatment options for the different inflammatory myopathies.