The idiopathic inflammatory myopathies (IIM) represent a heterogeneous group of muscle diseases. Given their potential amenability to immunosuppressive treatment, the early diagnosis of the IIM subtype and the exclusion of "myositis mimics" are of great importance. MRI can visualize muscle edema, fatty replacement, atrophy, subcutaneous pathology, and fasciitis. MRI furthermore provides information about the distribution of muscle involvement and help in guiding of muscle biopsy. This review discusses significant MRI developments aimed at improving the diagnostic sensitivity and specificity, and at assessing IIM activity and damage. The manuscript also highlights techniques such as magnetic resonance spectroscopy, blood oxygenation level-dependent MRI, and diffusion-weighted MRI and T2 mapping which may in the future allow to better characterize vascular involvement, metabolic reserves, dynamic muscle recruitment, and treatment outcome.