Abstract: Trastuzumab emtansine （T-DM1） is an antibody-drug conjugate. Through a stable linker， the HER-2 targeting antitumor properties of trastuzumab is conjugated with the cytotoxic activity of the microtubule-inhibitory agent DM1 （derivative of maytansine）. Efficacy has now been demonstrated in randomized trials as first-line， second-line， and later than the second-line treatment of advanced breast cancer. The NCCN panel recommends T-DM1 as a preferred option for treatment of patients with HER2-positive metastatic breast cancer who have previously received a trastuzumab-based regimen. Ongoing studies are also evaluating the use of T-DM1 in neoadjuvant， and adjuvant settings and in combination with other agents. In this article， we review the mechanism of action of T-DM1， efficacy and safety data in critical clinical studies， predictors of response or resistance to T-DM1 and the strategies to overcome resistance， may offer practical reference for clinicians.