Myositis, or the idiopathic inflammatory myopathies (IMM) are a group of rare autoimmune diseases of considerable health significance. Features include muscle weakness, raised skeletal muscle enzymes, and characteristic histopathological changes. IIM can be split into polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM). If diagnosed promptly PM and DM can be treated with immunosuppression, but IBM currently cannot be treated. Summarizing the current understanding of the epidemiology, genetic and environmental risk factors, and clinical features, this handbook gives practical strategies for laboratory investigations and treatment paradigms. Future strategies and ongoing trials are also discussed.