Trastuzumab emtansine (T-DM1) is a novel HER2-directed antibody-drug conjugate. T-DM1 consists of the potent antimicrotubule agent DM1, linked via a noncleavable linker to the HER2-specific monoclonal antibody trastuzumab. Preclinical studies demonstrate that T-DM1 has dual mechanisms of action: selective delivery of DM1 to the HER2-positive (HER2(+)) tumor cell combined with trastuzumab's activation of antibody-dependent cell-mediated cytotoxicity and inhibition of HER2-mediated signal transduction. In phase II studies, T-DM1 was active in patients with trastuzumab- and lapatinib-refractory metastatic breast cancer and led to improved progression-free survival compared with the combination of trastuzumab and docetaxel in the first-line setting. In a recent phase III trial in patients with metastatic breast cancer who previously received trastuzumab and a taxane, T-DM1 resulted in improved progression-free and overall survival compared with capecitabine and lapatinib. T-DM1 is associated with a favorable toxicity profile; reversible thrombocytopenia and hepatic transaminase elevations are the only grade ≥3 adverse event present in 5% or more of patients. Alopecia, peripheral neuropathy, and neutropenia are distinctly uncommon. On the basis of its improved efficacy and toxicity compared with capecitabine/lapatinib, T-DM1 should be considered the standard for patients with HER2(+) metastatic breast cancer who have previously progressed on trastuzumab and a taxane. Results from additional randomized studies in metastatic breast cancer are pending, and trials in the (neo)adjuvant setting are being initiated.