Due to the increasing research into familial clustering of cancer entities, more and more genes are being identified in which mutations explain this clustering. Mutations in the cadherin 1 (CDH1) and catenin alpha 1 (CTNNA1) genes are considered to be causative for the occurrence of hereditary diffuse gastric cancer. Those affected show an incidence of gastric cancer of around 40% up to the age of 80 years and affected women show an incidence of 55% for the occurrence of lobular breast cancer. In 2020 updated international guidelines were published for the clinical management of patients with hereditary diffuse gastric cancer. When the specific test criteria are fulfilled, patients should undergo genetic testing for mutations in the CDH1 and CTNNA1 genes. In cases of the familial occurrence of diffuse gastric cancer and detection of a pathological mutation, a prophylactic total gastrectomy with D1 lymphadenectomy is recommended. Alternatively, or when pathological mutations are not detected, a gastroscopy should be performed annually with targeted and random biopsies. The occurrence of lobular breast cancer should be monitored annually by magnetic resonance imaging (MRI) from the age of 30 years onwards. A bilateral mastectomy for risk reduction should be discussed in a multidisciplinary setting.