As HER2 testing in gastric cancer is different from that in breast cancer, the Working Group Molecular Pathology, a section of the Belgian Society of Pathology in collaboration with the Belgian Club of Digestive Pathology, the Belgium Group of Digestive Oncology and the Belgium Society of Medical Oncology, formed a committee to establish Belgian guidelines for HER2 testing in gastric cancer. These guidelines are based on the international recommendations and take the Belgian criteria for reimbursement of trastuzumab into account. The aim of these guidelines is to report the consensus of when and how HER2 testing should be performed in gastric cancer in order to establish strategies for ensuring optimal performance, interpretation and reporting assays.