Trastuzumab emtansine (T-DM1) is a novel antibody-drug conjugate, comprised of a potent cytotoxic drug connected via a stable linker to the anti-HER2 antibody, trastuzumab, thereby primarily targeting chemotherapy delivery to cells overexpressing the HER2 receptor. A Phase II randomized trial of T-DM1 in the front-line metastatic breast cancer setting revealed promising activity and improved safety compared with standard chemotherapy plus trastuzumab. Subsequently, a Phase III trial in patients with trastuzumab-pretreated metastatic breast cancer showed T-DM1 to be associated with prolonged progression-free and overall survival compared with lapatinib plus capecitabine. T-DM1 represents a major shift in the treatment of patients with breast cancer as it replaces traditional nontargeted chemotherapy with a 'smart' medication that directs the cytotoxic therapy to cancer cells by using a known biomarker.