Myositis, also known as idiopathic inflammatory myopathy, is a group of autoimmune diseases characterized by muscle weakness and muscle inflammation, including subtypes such as dermatomyositis, polymyositis, inclusion body myositis, and childhood dermatomyositis. There are two peak incidences of myositis, namely between 10 and 15 years of age or between 45 and 60 years of age. The etiology of myositis is currently unknown; genetic, infectious, and environmental factors are associated with the development of the disease, and abnormal immune inflammation is its pathological feature, caused by the autoimmune system attacking the body’s own cells, but the specific pathogenesis remains unclear. The clinical manifestations of myositis are diverse, mainly including muscle symptoms and skin symptoms. The former is dominated by muscle pain and pressure, muscle weakness, mostly accompanied by difficulty in squatting, weakness in lifting both arms, weakness of neck muscles, etc. Some patients may affect the esophageal muscles and have difficulty in swallowing. The latter is mainly manifested by rashes, such as edematous purplish red spots around the eyes, “V” shaped red rashes on the front of the neck and upper chest, and papules on friction-prone areas such as the interphalangeal joints of both hands and the extensor surfaces of elbows and knees. In addition, multi-system damage, such as interstitial lung fibrosis, cardiac arrhythmia, proteinuria, hematuria, etc., may occur, and malignant tumors may be associated. The clinical manifestations, treatment and prognosis of different subtypes of myositis are very different, and those with combined important organ damage and combined malignant tumors are more severe and have a poorer prognosis, even death. Therefore, early diagnosis, systematic and standardized treatment is the key to keep the condition stable. If symptoms of myositis are suspected, you should immediately visit the rheumatology department to improve the relevant examination.