The myositis syndromes, the most common forms of which are polymyositis and dermatomyositis, are defined by idiopathic chronic inflammation in skeletal muscle. Although initially described more than a century ago, these diseases are so rare and heterogeneous that we have only a limited understanding of their causes and treatment. Recently, autoimmune responses to nuclear and cytoplasmic autoantigens that are unique to patients with myositis, the myositis-specific autoantibodies, have proved clinically useful in helping predict signs and symptoms of myositis, immunogenetics, responses to therapy, and prognosis. We summarize this new information on the variety and nature of these autoantibodies, their target epitopes, and their possible use in identifying causes, pathogenetic mechanisms, and better therapies for these increasingly recognized disorders.