Gastric cancer is constituted by two histomorphological entities ''intestinal'' and ''diffuse'' that differ in epidemiology, pathogenesis, clinical outcome and genetic profile. Two distinct molecular pathways of genomic destabilization have been identified in gastric carcinogenesis: the microsatellite mutator phenotype (MMP) and a phenotype associated with chromosomal and intrachromosomal instability (CIN). The microsatellite mutator phenotype is caused by mismatch repair (MMR) deficiency and is associated with mutational inactivation; this condition is identified as microsatellite instability (MIN). CIN is characterized by chromosomal rearrangements and losses or gains of chromosomes, which in turn can induce oncogene activation and/or tumour-suppressor-gene inactivation. Mounting evidence suggests that MMP alterations, DNA aneuploidy and expression of the products of cancer-related genes are early markers of cell transformation, and may serve to identify the genetic pathway of gastric carcinoma. However, the lack of a clear genetic basis lends weight to the notion that gastric cancer may be affected by exposure to environmental factors. Helicobacter pylori is one of the most frequent infections worldwide and is the most important environmental risk factor associated with sporadic gastric cancer. Exposure of gastric epithelial cells to H. pylori results in the generation of reactive oxygen species and an increased level of inducible nitric oxide synthase that in turn may cause genetic alterations leading to cancer in a subset of subjects. In conclusion gastric cancer is the result of an interplay between genetic and environmental factors. The new technologies for the molecular analysis will be a useful tool to understand the individual's risk and settle novel biological therapeutic strategies.