Dermatomyositis and polymyositis are rare chronic inflammatory disorders with significant associated morbidity and mortality despite treatment. High-dose corticosteroids in addition to other interventions such as immunosuppressants, immunomodulators, and more recently, biologics are commonly used in clinical practice; however, there are no clear guidelines directing their use. Our objective was to systematically review the evidence for immunotherapy in the treatment of dermatomyositis and polymyositis. Relevant studies were identified through Embase and PubMed database searches. Trials were selected using pre-determined selection criteria and then assessed for quality. Randomized controlled trials and experimental studies without true randomization and including adult patients with definite or probable dermatomyositis or polymyositis were evaluated. Any type of immunotherapy was considered. Clinical improvement, judged by assessment of muscle strength after 6 months, was the primary outcome. Secondary outcomes included IMACS definition of improvement, improvements in patient and physician global scores, physical function, and muscle enzymes. Twelve studies met eligibility criteria. Differences in trial design, quality, and variable reporting of baseline characteristics and outcomes made direct comparison impossible. Although no treatment can be recommended on the basis of this review, improved outcomes were demonstrated with a number of agents including methotrexate, azathioprine, ciclosporin, rituximab, and intravenous immunoglobulin. Plasmapheresis and leukapheresis were of no apparent benefit. More high-quality randomized controlled trials are needed to establish the role of immunosuppressive agents in the treatment of these conditions and the clinical context in which they are most likely to be beneficial.