Dermatomyositis and polymyositis are treatable disorders of skeletal muscle. Despite their clinical similarities, they appear to have fundamentally different autoimmune origins. Inclusion body myositis, from its origins 30 years ago, has emerged as the commonest acquired myopathy of the elderly. Despite inflammatory changes, it is unclear whether it should be considered a primary inflammatory myopathy, and it generally responds poorly to the same treatments that are effective in other inflammatory myopathies.