Dermatomyositis (DM) is an idiopathic, inflammatory myopathy characterized by dermatologic changes, myopathy, some typical serologic findings, and frequent extramuscular involvement. Both children and adults may be affected, with certain features distinct to children. Dermatologic changes may occur without myositis in patients with amyopathic DM. DM has been reported to occur in patients undergoing treatment with certain therapeutic agents or with concomitant infections. DM may be associated with malignant disease in older patients. Newer serologic tests may identify subsets with different clinical patterns and prognoses. Magnetic resonance imaging is increasingly used to identify muscle involvement and to select sites for biopsies. Newer studies provide improving guidance for therapies in steroid nonresponders.