Amplification of the HER-2 gene occurs in approximately 25% of breast cancers, causing up-regulation of key signaling pathways which control cell growth and survival. In breast cancer patients, HER-2 overexpression correlates with an aggressive phenotype and poor prognosis. HER-2, therefore, has become the focus of many anti-cancer therapeutic approaches. Trastuzumab (Herceptin), a humanized monoclonal antibody directed against the extracellular domain of HER-2, was the first FDA-approved HER-2-targeted therapy for the treatment of metastatic breast cancer. However, not all HER-2-overexpressing patients respond to trastuzumab and most that initially respond develop resistance within one year of treatment. Trastuzumab resistance has been studied in cell line models of resistance and several mechanisms of resistance have been proposed. More recent anti-HER-2 strategies involve targeting its tyrosine kinase domain; for example, lapatinib (Tykerb) is a dual HER-2 and EGFR tyrosine kinase inhibitor and has shown efficacy as a single agent and in combination with other therapeutics. A number of novel HER-2 antagonists are currently in preclinical or clinical development, including both monoclonal antibodies and small molecule inhibitors. Increased understanding of HER-2 signaling in breast cancer, and of response and resistance to HER-2 antagonists, will aid the development of strategies to overcome resistance to HER-2 targeted therapies.