CONTEXT: Overexpression of human epidermal growth factor receptor 2 (HER2) is an adverse prognostic parameter for patients with breast cancer. Patients with HER2-positive tumors are, however, likely to gain significant benefit from targeted therapy with trastuzumab. Four large-scale trials have assessed the efficacy and safety of adjuvant trastuzumab. In all 4 trials, disease-free survival and overall survival were significantly improved with adjuvant trastuzumab (36%-52% and 33%-41%, respectively), and improvements were observed in all patient subgroups. OBJECTIVE: To describe, in detail, the clinical benefits of using trastuzumab in the adjuvant setting, and to emphasize the importance this places on the pathologist to correctly identify all patients with HER2-positive tumors with a high degree of accuracy. DATA SOURCES: Published literature on both HER2 testing and the adjuvant use of trastuzumab. CONCLUSIONS: Immunohistochemistry and/or fluorescence in situ hybridization are routinely used to determine HER2 status. Maintaining quality assurance throughout a standardized testing process is essential to achieve accurate and reproducible assay results. Adherence to the new American Society of Clinical Oncology/College of American Pathologists HER2 testing guidelines will help to ensure correct identification of all patients who may benefit from adjuvant trastuzumab and has significant implications for patient outcomes.