Overexpression, or gene amplification, of the human epidermal growth factor receptor 2 (HER2) is evident in 20% to 25% of breast cancers. The biologic agent trastuzumab is an HER2-targeted monoclonal antibody that inhibits the proliferation of tumor cells and induces tumor cell death through multiple mechanisms of action. Currently, trastuzumab is approved for use in the adjuvant and metastatic settings. Trials combining trastuzumab with neoadjuvant chemotherapy suggest that patients with HER2-positive breast cancer also may benefit from preoperative trastuzumab. For this article, the author reviewed efficacy and safety data from key studies of patients who received neoadjuvant trastuzumab-based therapy. Studies were identified from literature searches of publication and congress databases. The results of 3 large phase 3 trials (the M. D. Anderson Cancer Center neoadjuvant trastuzumab trial, the Neoadjuvant Herceptin [NOAH] trial, and the German Breast Group/Gynecologic Oncology Study Group "GeparQuattro" trial) demonstrated that, compared with chemotherapy alone, neoadjuvant trastuzumab plus chemotherapy significantly increased pathologic complete response rates to as high as 65%. Improvements in disease-free, overall, and event-free survival also were reported in the NOAH trial. In addition to demonstrated efficacy, a low incidence of cardiac dysfunction suggests that neoadjuvant trastuzumab is both effective and well tolerated. Similar results have been reported in a range of phase 2 studies using different trastuzumab-based regimens. These encouraging data led the National Comprehensive Cancer Network to recommend treating patients who have operable, locally advanced, HER2-positive breast cancer with neoadjuvant paclitaxel plus trastuzumab followed by 5-fluorouracil, epirubicin, and cyclophosphamide plus trastuzumab.