This chapter reviews the association between cancer and the idiopathic inflammatory myopathies (IIM), which includes dermatomyositis (DM), antisynthetase syndrome (ASyS), immune-mediated necrotizing myopathy (IMNM), and inclusion body myositis (IBM). Accumulating evidence shows that the risk of a coexisting malignancy is high in patients with DM, especially among those with anti-Tif1γ autoantibodies. Patients with IMNM and no defined autoantibodies also have an increased risk of malignancy. Recent evidence demonstrates that many IBM patients have increased numbers of circulating CD57+ CD8+ T cells, consistent with a diagnosis of large granular lymphocytic leukemia. In contrast, IMNM patients with anti-SRP or anti-HMGCR autoantibodies as well as patients with ASyS syndrome do not have a definitively increased risk of cancer. Patients who have a cancer treated with one of the immune checkpoint inhibitors can develop myositis (ICI-myositis), sometimes along with myasthenia gravis and/or myocarditis.