Helicobacter pylori is the major cause of chronic gastritis. The predominant anatomic distribution of the gastritis is antral in the majority of individuals. In a small minority, the corpus is predominantly involved. The former pattern is associated with duodenal ulceration in some patients, but the majority of those infected never develop either symptoms or disease. The latter form is associated with the development of gastric ulcer and carcinoma and may be protective against the development of Barrett's esophagus. It is the physiological changes associated with the histological changes and the, as yet poorly, defined host response, which are of paramount importance in determining the evolution of a disease or whether the infected individual remains asymptomatic and disease free. This article addresses the various relationships between H. pylori infection, histology, gastric physiology, and disease.