Although the etiology of gastric cancer and the mechanisms involved in its carcinogenesis are still poorly understood, dietary habits and life style as well as bacterial infections have been suggested to be important in the tumorigenic process. However, there is also an increasing evidence that a genetic predisposition in at least a subset of patients, plays an important role. Germline mutations in the E-cadherin gene have been described to be the molecular genetic cause of an hereditary diffuse type gastric cancer syndrome. In addition, gastric cancer is observed more frequently in association with some hereditary tumor syndromes which are mainly characterized by tumors in other organs. This article will summarize recent findings about the hereditary diffuse type gastric cancer syndrome and about gastric cancer in association with hereditary tumor syndromes with a known molecular genetic basis.