Stomach cancer is one of the most common malignant diseases in human population. There are almost one million of newly diagnosed cases each year. Five year survival is only about 20%. There are still no standardized or generally accepted chemotherapy protocols for its treatment. Since the disease is usually diagnosed at a very advanced stage, treatment is only palliative. Therefore, it is necessary to find and to introduce new therapeutic options which could extend the survival of patients with locally advanced or metastatic gastric cancer. About 15% of stomach carcinomas are positive for HER2. HER2 is a molecule which was found at the cell surface of the different human malignancies including gastric cancer. It represents target molecule that can be blocked by use of monoclonal antiHER2 antibodies. In 2010 the results of the first, international and randomized study about antiHER2 therapy of metastatic gastric carcinoma were published. The study showed regression of the primary tumor, prolongation of survival and prolongation of the relapsing time in the group of patient that had been treated with combination of chemotherapy and antiHER2 antibody in comparison with patients that had been treated with chemotherapy only. The very same year HER2 testing for gastric cancer was introduced into routine. In 2012. first guidelines about proper testing protocol were published. Very soon it became clear that the application of guidelines, in the meaning of HER2 testing at the one section of the resected tumor or at the endoscopic material, results in too many false negative tumors. There is an opinion that endoscopic material cannot be used as an equivalent for HER2 testing at the whole section. Also, testing at the one tumor block is not necessarily enough because of known HER2 heterogeneity of gastric cancer. The results of our study show that the problem of HER2 heterogeneity of gastric cancer could be »bypassed« by the use of tissue microarrays. Technology of tissue microarrays would enable the testing of larger volume of tumor tissue within short period of time and in significantly lower cost in comparison with testing at the whole sections of the tumor tissue.