SUMMARY: Besides surgery, radiation, chemotherapy, and endocrine treatment, immunotherapy has become an established part of systemic therapy in treating metastatic breast cancer. One of the most interesting targets for the design of anticancer therapeutics is the HER2/ErbB2 receptor which is overexpressed in about 20-25% of breast cancers. Given the poor prognosis of women whose tumors express ErbB2 (HER2) at high levels, accurate determination of the ErbB2 status should be routinely performed in women with newly diagnosed invasive breast cancer. Efficacy and safety data of numerous trials led to the approval of the monoclonal antibody trastuzumab as the first ErbB2-targeting therapy in ErbB2-positive breast cancer. However, the majority of patients who achieve an initial response to trastuzumab-based regimens for metastatic disease develop resistance within 1 year. This underlines the need for alternative or additional anti-ErbB2-targeting strategies.