A multifactorial and multistep model of gastric cancer (GC) is currently accepted, according to which different environmental and genetic factors are involved at different stages in the cancer process. The aim of this article is to review the most relevant information published on the relative contribution of genetic and environmental factors. Large meta-analyses confirmed the association between IL8, IL10, TNF-b, TP53 and PSCA, while genetic variation at different genes such as XPG, PLCE1, HFE, ERCC5, EZH2, DOC2, CYP19A1, ALDH2, and CDH1 have been reported to be associated with GC risk. Several microRNAs have also been associated with GC and their prognosis. Cohort studies have shown the association between GC and fruit, flavonoid, total antioxidant capacity, and green tea intake. Obesity was associated with cardia GC, heme iron intake from meat with GC risk. Several large meta-analyses have confirmed the positive association of GC with salt intake and pickled foods and the negative association with aspirin use.