Using the standard slide-based techniques of immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH), it has been firmly established that human epidermal growth factor receptor 2 (HER2) is overexpressed in adenocarcinoma of the upper gastrointestinal tract. In the ToGA trial, the addition of the monoclonal antibody trastuzumab to a standard regimen of cisplatin and fluoropyrimidine resulted in a clinically and statistically significant benefit in terms of response rate, median progression-free survival, and median overall survival in HER2-positive patients. Major differences exist, however, between HER2 testing in gastric/gastroesophageal junction (GEJ) cancer versus breast cancer, and the ToGA trial employed a significant modification of the breast cancer scoring criteria. As trastuzumab approaches regulatory approval in the United States for gastric/GEJ cancer, it is critical that pathologists and diagnostic laboratories learn and apply the unique criteria for assessing gastric/GEJ tumors for their HER2 status defined by the ToGA investigators, as they ready themselves for the approximately 50,000 new specimens that will be tested for HER2 status by both IHC and FISH each year.