Idiopathic inflammatory myositis (IIM) is classified into 4 subtypes based on clinical and histopathological features. Polymyositis and dermatomyositis (PM/DM) are 2 of those subtypes in addition to immune-mediated necrotizing myopathy and inclusion body myositis. The presence of autoantibodies and inflammatory infiltration in the muscles suggests that idiopathic inflammatory myositis is an autoimmune condition, but the target autoantigens have not yet been identified. Apart from symmetric proximal muscle weakness, polymyositis and dermatomyositis have several clinical manifestations in common when compared to inclusion body myositis and necrotizing myopathy and can be grouped for discussion. The most commonly used criteria for PM/DM are Peter/Bohan Criteria.[1]: 1. Symmetric proximal muscle weakness. 2. Elevated serum muscle enzymes. 3. Myopathic changes in electromyography (EMG). 4. Characteristic muscle biopsy abnormalities. 5. Typical rash of dermatomyositis for diabetes mellitus.