The majority of gastric adenocarcinomas are related to chronic inflammation induced by Helicobacter pylori infection. For intestinal-type gastric cancer, a multistep process of mucosal alterations leading from gastritis via glandular atrophy, intestinal metaplasia and dysplasia to invasive carcinoma is well recognized. Ongoing clinical studies focus on a ‘point of no return’. It is defined as a situation when certain alterations are no longer reversible by H. pylori eradication and progression to gastric cancer may continue. H. pylori affects the mucosal as well as the systemic immune response by secretion of cytokines and the recruitment of distinct inflammatory cells. The immune response is characterized by a balance between a Th1-dominated response and the recruitment of antigen-specific regulatory T cells that allow the bacteria to persist in human gastric mucosa. Besides immune-mediated effects, H. pylori induces cellular alterations as well as genetic alterations in genes that are essential for the epigenetic integrity and mucosal homeostasis. These genetic alterations during gastric cancer development are in focus of intensive research and should ultimately allow the identification of risk factors involved in gastric carcinogenesis. The detection of individuals at high risk for gastric cancer would help to design appropriate strategies for prevention and surveillance.