Helicobacter pylori is present in the stomach of more than half of the world population. Based on compelling epidemiological evidences, it was classified by the World Health Organization as a type I gastric carcinogen. It is generally believed that gastric cancer development is a multi-step progression from chronic gastritis to atrophy, intestinal metaplasia, dysplasia, and cancer. Individuals infected with H. pylori have at least a 2-fold increase in risk of gastric cancer development though only a small proportion of infected individuals will ultimately develop this malignancy. The exact mechanisms underlying how H. pylori triggers or causes gastric cancer remain elusive. Certain H. pylori genotypes like cagA, vacA s1 or babA1 are considered to be of higher virulent potential. Apart from the bacterial factors, the host response to chronic H. pylori infection may also attribute to the cancer risk. It was found that individuals who carry pro-inflammatory cytokine gene polymorphism have a substantial increase in risk of cancer development. The combination of bacterial and host genotypes may have a synergistic effect on cancer development. Despite the strong causal link between chronic H. pylori infection and gastric cancer, the role of H. pylori eradication in preventing gastric cancer remains controversial. More long-term data may be necessary to clarify this controversy.