Abstract: The 2003 WHO classification of breast tumor histology represented the advances in breast tumor histology and new techniques used in diagnosis, histological typing and forecasting prognosis. Some new histological types were nominated. The new classification system emphasized the combination of clinic and pathology. pTNM stages and prognostic factors were also added. It mentioned the significance of histological grading and expression of ER, PR and Her- 2, and defined ductal intraep-ithelial neoplasia. Invasive carcinoma was relabeled carcinoma, and ductal carcinoma in situ was relabeled ductal intraepithelial neoplasia. Lobular carcinoma in situ was reclassified as lobular neoplasia.The diagnosis of suspicious malignant lesions should be downgraded to avoid the harm to patients' physical and mental state of overdiagnosis and overtreatment. The new classification system will act as a reference instead of an imperative. In spite of the merits above, deficiencies still existed, such as poor systematics and irrationality in grades and classifications of some tumors. Because some frequently seen lesions are not included in the new classification, pathologists may feel confused in the process of diagnosis. In clinical practice, we should combine the new classification with our practical experience to benefit patients.