Among the suspected bacterial causes of cancer, H. pylori is the agent more consistently linked to malignancy. After its discovery in 1983 and the later confirmation as the leading cause of chronic gastritis, several studies were performed to prove an association between H. pylori infection and gastric carcinoma. The epidemiological data have been so strong that in 1994 the International Association for Research on Cancer stated that “there was sufficient evidence” to classify H. pylori as a group I carcinogen in humans. However, the exact mechanisms underlying the link between H. pylori infection and gastric carcinoma remain still to be elucidated. The natural history of H. pylori infection shows that, although roughly half of the world’s human population bears the organism, only a minority of individuals develop clinically important outcomes (e.g. peptic ulcer, lymphoproliferative diseases, atrophic gastritis and gastric carcinoma): host’s genetic make-up, duration of infection, diet and differences between H. pylori strains have been proposed as factors potentially able to influence the outcomes in different individuals. The damaging agents by which H. pylori could promote gastric carcinogenesis are produced either by the organism or as a consequence of the host inflammatory response to the infection. Gastric mucosal chronic damage may, therefore, lead to changes in the pattern of epithelial cell kinetic (increase in cell proliferation and induction of apoptosis) in gastric glands which may induce DNA injury with irreversible genetic lesions. Finally, a direct association between H. pylori infection and the induction of gastric carcinoma has been recently demonstrated in an animal model, giving further credence to the role of this organism in gastric carcinogenesis.