Gastric cancer is a leading cause of cancer deaths, but analysis of its molecular and clinical characteristics has been complicated by histological and aetiological heterogeneity. Stomach cancers are 90% adenocarcinoma; lymphoma, carcinoid, and stromal tumours may occur. Adenocarcinoma can be subdivided into histological Lauren and the World Health Organization (WHO) classifications, but this information has not led to the development of histologic subtype-specific treatment options. One way to potentially improve treatment for gastric cancers is to better understand the pathogenesis of this disease, the contribution of Helicobacter pylori infection, and host immune response to lead to the development of integrated histological and molecular classification schemes for gastric cancer. The hope is that these studies may facilitate the development of clinical trials to explore therapies in defined sets of patients, ultimately improving survival from this deadly disease.