Despite a major decline in incidence and mortality over several decades, stomach cancer is still the fourth most common cancer and the second most common cause of cancer death in the world. There is a 10-fold variation in incidence between populations at the highest and lowest risk. The incidence is particularly high in East Asia, Eastern Europe, and parts of Central and South America, and it is about twice as high among men than among women. Prognosis is generally rather poor, with 5-year relative survival below 30% in most countries. The best established risk factors for stomach cancer are Helicobacter pylori infection, the by far strongest established risk factor for distal stomach cancer, and male sex, a family history of stomach cancer, and smoking. While some factors related to diet and food preservation, such as high intake of salt-preserved foods and dietary nitrite or low intake of fruit and vegetables, are likely to increase the risk of stomach cancer, the quantitative impact of many dietary factors remains uncertain, partly due to limitations of exposure assessment and control for confounding factors. Future epidemiologic research should pay particular attention to differentiation of stomach cancer epidemiology by subsite, and to exploration of potential interactions between H. pylori infection, genetic, and environmental factors.