Adenocarcinomas of the esophagus and stomach constitute a substantial number of cancer cases worldwide. Most patients in the United States are diagnosed at an advanced or metastatic stage and, therefore, the prognoses have been poor. New treatments are needed to augment standard surgical and medical management. Recent studies have shown that a subset of esophageal and gastric adenocarcinomas overexpress the HER2 protein, similar to the overexpression seen in breast cancer. Because trastuzumab, a monoclonal antibody to the HER2 receptor, has been used with success in primary and HER2 positive metastatic breast cancers, the phase III ToGA trial was designed to assess the impact of trastuzumab in patients with HER2 positive gastric cancers. They have reported an increase in overall survival time for patients treated with chemotherapy and trastuzumab compared to those treated with chemotherapy alone. They have reported an increase in overall survival time for patients treated with chemotherapy and trastuzumab compared to those treated with chemotherapy alone. This means that accurate HER2 testing in gastric and esophageal carcinomas is necessary. While the breast cancer scoring system can be used to determine HER2 status in most cases, modifications are necessary to accommodate the heterogeneity and incomplete membrane staining that are observed more frequently in gastric cancers. An understanding of the scoring modifications is required for proper stratification of gastric cancer patients for treatment.