The idiopathic inflammatory myopathies (IIMs) of childhood are a heterogenous group of rare diseases characterized by chronic skeletal muscle inflammation. Although juvenile dermatomyositis is the most common of these disorders, children may develop any of the other types of myositis that have been better studied in adults. These include not only the IIMs delineated by clinicopathologic features but also the serologic groups of myositis, recently defined by myositis-specific and myositis-associated autoantibodies. Differences in the frequencies of IIM groups between children and adults suggest differences in exposures to myositis-inducing environmental agents or differences in the frequencies of susceptibility genes between these two populations. Further investigation of the heterogeneity of the childhood IIMs and of the newly described clinicopathologic and autoantibody groups should improve our understanding and treatment of these disorders.