Chronic gastritis is a common disease which forms an important background to the pathogenesis of several gastric diseases. In most instances, gastritis seems to be a bacterial (microbial) disease. It begins as long-lasting, chronic inflammatory reaction directed against Helicobacter pylori (HP), or occasionally against other spiral bacteria, which colonize in the space between the surface epithelium and the mucous layer. Gastritis may, irrespectively of the HP-related or HP-independent origin, progress to an atrophy (chronic gastritis with atrophy) in the underlying mucosa. Prevalence of gastritis increases with increase in age, but great variations exist in the age-specific prevalence and in mean age of onset of the gastritis in different populations. A high rate and an early onset of the HP-related gastritis associates with low socio-economic status. Chronic gastritis, and the gastritis with atrophy in particular, may interfere with the function of the affected gastric mucosa, and may subsequently increase or decrease the risk of some gastric diseases, such as cancer and peptic ulcer. Both antral and corpus gastritis with coexistent severe atrophic changes have been shown to be associated with an increased risk of gastric cancer. In addition, gastritis seems to also play an important role in the pathogenesis of peptic ulcer. Virtually all patients with DU and GU have coexisting and preceding gastritis. The cumulative risk of ulcer has been estimated to be high in subjects with gastritis, but, in contrast, to be low in subjects who have normal gastric mucosa.