Dermatomyositis (DM) and polymyositis (PM) are autoimmune myopathies characterized clinically by proximal muscle weakness, muscle inflammation, extramuscular manifestations, and frequently, the presence of autoantibodies. Although there is some overlap, DM and PM are separate diseases with different pathophysiological mechanisms. Furthermore, unique clinical phenotypes are associated with each of the myositis-specific autoantibodies (MSAs) associated with these disorders. This review will focus on the clinical features, pathology, and immunogenetics of PM and DM with an emphasis on the importance of autoantibodies in defining unique phenotypes and, perhaps, as clues to help elucidate the mechanisms of disease.