Third-generation aromatase inhibitors(AIs)have recently overtaken tamoxifen(TAM)to play a key role in the adjuvant treatment of post-menopausal hormone receptor-positive breast cancer patients. The accumulating evidence of neoadjuvant chemotherapy is now contributing to growing interest in neoadjuvant endocrine therapy using AIs. Recent research has revealed that administration of AIs 3-4 months prior to surgery is more effective than TAM in terms of the clinical response rate and the breast-conserving surgical procedure rate. In addition, the Preoperative Endocrine Prognostic Index(PEPI), which uses four factors, namely MIB1/Ki67 levels, pathological tumor size, and lymph node and ER status following neoadjuvant endocrine therapy, has been demonstrated very effective for predicting the prognosis of patients treated with AIs. Investigation of the different efficacies of various AIs, the optimal dosing period, and suitable combination with adjuvant chemotherapy will enable breast cancer drug therapy to be individualized to suit each patient's condition.