Idiopathic inflammatory myopathies (IIM) are diseases that are potentially amenable to immunomodulatory therapy. The challenge for the neuropathologist consists in distinguishing these myopathies from secondary inflammatory myopathies, especially in the context of some muscular dystrophies and metabolic diseases that may also show inflammatory infiltrates. There are generalized IIMs (dermatomyositis, polymyositis, sporadic inclusion body myositis) and focal ones (e.g., proliferative myositis, macrophagic myofasciitis). This review provides diagnostic criteria for each of these and includes pathogenetic mechanisms where available.