Trastuzumab emtansine (T-DM1), a novel drug developed for the treatment of HER2-positive breast cancer, is a human epidermal growth factor receptor (HER2) targeted antibody drug conjugate, composed of trastuzumab, a stable thioether linker, and the potent cytotoxic agent DM1 (derivative of maytansine). It has been shown that, in preclinical studies, it has anti-tumor activity in trastuzumab refractory cancer cells. In this review, we aim to show the clinical data about trastuzumab-DM1 (T-DM1) therapy and to discuss the therapy advantages for the management of patients with HER2-positive breast cancer. T-DM1 showed positive results in clinical studies of HER2-positive metastatic breast cancer. PubMed database, ASCO and San Antonio Breast Cancer Symposium Meeting abstracts were searched up to September 2012 by using the terms ‘trastuzumab emtansine (T-DM1) and anti-HER2 treatment’; papers which were considered relevant for the aim of this review were selected by the authors. The phase III randomized trial EMILIA has shown that T-DM1 provided objective tumor responses and significantly improved progression free survival and overall survival compared to lapatinib and capacitabine combination in HER2-positive metastatic breast cancer patients treated with a prior taxane and trastuzumab regimen. It is believed that T-DM1 will play a role in the management of patients with advanced and early stage HER2-positive breast cancer, but this awaits further study. In particular, the ongoing phase III trials MARIANNE and TH3RESA will further give information about the place of T-DM1 in the treatment algorithms for HER2-positive disease. The trials of T-DM1 as a single agent and in combination with other chemotherapies have shown clinical activity and a favorable safety profile in patients with HER2-positive metastatic breast cancer. There are ongoing studies of T-DM1 showing an increasing tendency towards moving the study of these agents to earlier stages of HER2-positive breast cancer.