Human epidermal growth factor receptor 2 (HER2) is a transmembrane growth factor receptor with tyrosine kinase activity regulating cell growth and survival. HER2 amplification is responsible for protein overexpression with a negative impact on prognosis, but HER2-overexpressing patients could be treated with anti-HER2 therapy. In gastric cancer HER2 overexpression is observed in approximately 22% of patients, but prognostic significance in this context is not clear. In colon adenocarcinoma, HER2 gene amplification is reported in a wide range from 0% to 83%. The wide range of reported series could be essentially attributable to a difference in scoring systems for HER2 protein expression. The use of testing score, although measuring the same protein, is different from gastric cancer to colorectal cancer. Herein the different scoring models are reported.