Immune-mediated necrotizing myopathy (IMNM) is a group of inflammatory myopathies that was distinguished from polymyositis in 2004. Most IMNMs are associated with anti-signal recognition particle (anti-SRP) or anti-3-hydroxy-3-methylglutaryl-coA reductase (anti-HMGCR) myositis-specific autoantibodies, although ~20% of patients with IMNM remain seronegative. These associations have led to three subclasses of IMNM: anti-SRP-positive IMNM, anti-HMGCR-positive IMNM and seronegative IMNM. IMNMs are frequently rapidly progressive and severe, displaying high serum creatine kinase levels, and failure to treat IMNMs effectively may lead to severe muscle impairment. In patients with seronegative IMNM, disease can be concomitant with cancer. Research into IMNM pathogenesis has shown that anti-SRP and anti-HMGCR autoantibodies cause weakness and myofibre necrosis in mice, suggesting that, as well as being diagnostic biomarkers of IMNM, they may play a key role in disease pathogenesis. Therapeutically, treatments such as rituximab or intravenous immunoglobulins can now be discussed for IMNM, and targeted therapies, such as anticomplement therapeutics, may be a future option for patients with refractory disease.