Clinical laboratory testing for human epidermal growth factor receptor 2 (HER2) status in newly diagnosed breast cancer is critically important for therapeutic decision making. Unlike most pathologic testing, which serves as an adjunct to establishing a diagnosis, the results of HER2 testing stand alone in determining which patients are likely to respond to trastuzumab, a monoclonal antibody against HER2. Given the significant clinical impact of the testing results on subsequent patient management, the accuracy, precision, and reproducibility of HER2 testing are critical. At present, several preanalytic factors, including the time from tissue removal to tissue fixation, are underappreciated as important variables that have the potential to negatively impact the consistency and reliability of HER2 testing. Rigorous quality control and standardization of the testing process, from the handling of tissue samples to interpretation and reporting of results, are essential for achieving accurate and reproducible assay results.