Therapies targeting the human epidermal growth factor receptor (HER) 2 are effective in metastatic breast cancer (MBC). We review the efficacy of HER2-directed therapies, focussing on monoclonal antibodies and tyrosine kinase inhibitors targeting HER2 that have been tested in phase II-III studies in MBC. Trastuzumab is an important component of first-line treatment of HER2-positive MBC. New anti-HER2 drugs have the potential to change clinical practice. The potential role of the different drugs and regimens is yet to be determined. The response rate for trastuzumab-DM1 of 26-64% is comparable to those obtained for capecitabine plus lapatinib (48%), continuing trastuzumab in combination with capecitabine (48%), pertuzumab plus trastuzumab (24%), and neratinib (24%). Strategies combining multiple HER2-directed therapies might yield additive or synergistic effects and lead to improved outcome. The future challenges include understanding HER2 functions, designing rational combinations and optimal selection of patients.