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The human epidermal growth factor receptor 2 (HER2) is overexpressed in 10% to 35% of gastric and gastroesophageal junction (GEJ) adenocarcinomas. In 2010, the phase III Trastuzumab for Gastric Cancer (ToGA) trial showed that addition of the anti-HER2 monoclonal antibody trastuzumab to chemotherapy significantly improved survival of patients with advanced or metastatic tumors that were positive for HER2 overexpression. As a result, HER2 testing is now recommended for all patients with advanced or metastatic disease, although there is still some debate as to the optimal methods of assessment. HER2 expression in gastric and GEJ tumors shows several differences compared with breast tumors and, for this reason, the proposed criteria for scoring HER2 expression in biopsies and resections of gastric and GEJ carcinomas differ from those used in breast carcinomas. This review discusses what is currently known about the patterns of HER2 expression in gastric and GEJ adenocarcinomas, summarizes the findings of the ToGA trial and its clinical implications, and provides an overview of the recommended guidelines for the most accurate evaluation of HER2 status in gastric and GEJ cancer.