The idiopathic inflammatory myopathies (IIMs), also known as myositis syndromes, are a collection of heterogeneous disorders that share the common feature of chronic muscle inflammation of unknown cause. These disorders may occur in adults or children and are sometimes associated with other connective tissue disorders and a variety of cancers. A combined clinical, laboratory, and pathologic evaluation is needed to establish the diagnosis of these acquired systemic connective tissue diseases to rule out the many disorders that mimic IIMs. This module reviews the classification of IIMs, including polymyositis, dermatomyositis, inclusion body myositis, myositis associated with other connective tissue diseases and cancer, and antisynthetase syndrome. The epidemiology; etiology, genetics, and environmental factors; pathophysiology and pathogenesis; diagnosis; differential diagnosis; treatment; and prognosis of IIMs are discussed. Tables describe the criteria for polymyositis, dermatomyositis, and inclusion body myositis; well-characterized subgroups of the IIMs in adults and children; presentation of polymyositis; differential diagnosis of muscle weakness or pain; features that assist in discriminating IIMs from other myopathies; goals for managing IIMs; and key factors for achieving adequate corticosteroid response in IIMs. Figures demonstrate skin findings in IIMs, muscle pathology of IIMs, magnetic resonance imaging of three patients with different IIMs, and treatment approaches to the management of myositis patients. This review contains 4 highly rendered figures, 8 tables, and 80 references.