Aromatase inhibition provides both paracrine/intracrine and endocrine treatment. Recent accumulated data clarified that 3rd generation aromatase inhibitors potently suppress intratumoral estrogen synthesis particularly in postmenopausal patients. In the 2nd-line treatment for metastatic breast cancer patients, aromatase inhibitors achieved results antitumor activity at least equal to and sometimes better than that of tamoxifen. In the first-line treatment for metastatic breast cancer patients, a recent pivotal study clearly demonstrated that aromatase inhibitor was superior to tamoxifen. Based upon these results, various adjuvant trials which compare aromatase inhibitors with tamoxifen and attempt to determine optimal combination therapies and treatment periods with aromatase inhibitors are currently being conducted. In addition, preliminary studies conducted in neoadjuvant setting indicated that aromatase inhibitors showed an extremely high response rate, which predicts a future paradigm, that neoadjuvant therapy using aromatase inhibitors singly or in combination may become standard for hormone-responsive and post-menopausal breast cancer patients.