The use of chemotherapy and endocrine therapies as adjuncts to the treatment of early-stage breast cancer has yielded small but significant improvements in disease-free and overall survival. Increased understanding of the role of growth factor receptors enabled the rational development of agents that are capable of modulating their function. A humanised monoclonal antibody to the HER2 receptor, trastuzumab, has demonstrable single-agent activity in metastatic breast cancer and enhances the antitumour effects of chemotherapy. As a consequence, trastuzumab has been tested in the adjuvant setting the results of which have been presented recently. This review briefly summarises the use of trastuzumab in advanced breast cancer and describes recent studies of its use in the adjuvant setting.