Carcinoma in situ of the breast counts as breast cancer and belongs to the early stage of breast cancer. According to histologic changes, it can be divided into intraductal carcinoma in situ and lobular carcinoma in situ. These two types of carcinoma in situ are confined within the basement membrane and have not infiltrated into the surrounding tissues and mesenchyme, nor invaded the peripheral blood vessels and lymphatic vessels to metastasize to distant places. Generally, they are treated by surgery and have better prognosis. The pathogenesis of breast cancer is still unclear and may be related to estrogen, radiation exposure, family genetics and environmental factors. It includes invasive carcinoma and non-invasive carcinoma, in which invasive carcinoma can be metastasized at early stage and usually requires surgery, radiotherapy, chemotherapy and targeted therapy. Carcinoma in situ has the possibility of developing into invasive cancer, and if metastasis occurs, the treatment effect is poorer and the prognosis is not as good as that of carcinoma in situ. Therefore, when in situ breast cancer is detected, go to regular hospitals as early as possible for a complete examination, and actively treat it according to the doctor’s comprehensive judgment so as to recover health as early as possible. Don’t delay the disease and miss the treatment.